Recommended low profile card: GIGABYTE GV-N75TOC-2GL G-SYNC Support GeForce GTX 750 Ti 2GB ($155) (Newegg link) ($155)
Recommended low profile, single slot card: VisionTek 900702 Radeon R7 250 1GB 128-Bit GDDR5 (Newegg link) ($100)
I built myself a low power mini-ITX system in an Antec ISK 300-150, with a Core i3-4130T (35W) as its heart and a Pico PSU 160W for power. Intel's IGP is sufficient for now, but I'd eventually like to be gaming on it at 1080p, with middling quality settings (lightweight stuff like Diablo 3, Team Fortress 2, etc). The ISK 300-150 presents an unfortunate dilemma for this, in that there's only 20mm of space from the center of a board's PCIe slot to the edge of the case - single slot video cards only. For height, the case takes low profile only. And of course, it can't require an additional PCIe power connector. So my options are few.
That said, as I've been looking at the available options I wanted to post something that might be useful to others with similar space requirements, or those who are just limited to low profile and don't have a width restriction (some OEM boxes, Dell, HP, etc, might be low profile but have room for multiple expansion cards). Some have a GPU preference, so I wanted to cover both options. So, here we are:
Low Profile - NVIDIA
GeForce GTX 750 Ti - 60W power draw and the full capabilities of GM107 (as far as we know, at least) makes this the best option for a low profile gaming card right now. And yes, there are low profile GeForce 750 Ti cards.
GIGABYTE GV-N75TOC-2GL G-SYNC Support GeForce GTX 750 Ti 2GB - 2GB of RAM and readily available at the egg, here's your fastest low profile video card currently available.
Newegg link ($155)
Amazon link ($160)
GeForce GTX 750 - with excellent performance for its segment and a mere 55W power draw, a few vendors have already pushed out low profile versions of the GTX 750. While a 750 Ti is going to get you better performance, maybe your budget is really tight, or maybe you're just going to be casually gaming, not enough to make the extra cost for the Ti version worthwhile. So here are a few 750 options:
ZOTAC GeForce GTX 750 1GB (ZT-70702-10M)
Newegg link ($120)
Amazon link ($120)
GIGABYTE GV-N750OC-2GL G-SYNC Support GeForce GTX 750 2GB - Gigabyte's version of the 750 in low profile comes with 2GB of RAM. it also carries a $20 price premium over the Zotac card - whether or not that is worth it is dependent on what you'll be doing with it, because for another $15 you can get their 2GB 750Ti (linked above). So where do you draw the line?
Newegg link ($140)
Low Profile - AMD
Radeon R7 250 - 384SPs, with GDDR5. Good memory bandwidth, about 800GFLOPS/sec of power... so the question becomes: is the 512SP card so bottlenecked by the slower DDR3 that it can't keep up with a lower-end card with faster memory? It's possible. There are a lot of single slot solutions for these cards, so scroll down to the "low profile, single slot" section for R7 250 cards. However, there is one specifically worth mentioning here.
Radeon R7 250 Core Edition - Low Profile (R7-250A-ZLF4)
A dual slot solution (incorrect heatsink height listed on their website specs, at the time of this writing it showed 14mm, actual height is about 36mm), with a shrouded heatsink/fan, so it's possible you'll get less fan noise and better cooling with this than you would with a single slot solution.
Newegg link ($103)
Amazon link ($93)
For other R7 250 cards, the rest of which I have listed are single slot, see below.
Radeon HD 7750 - you can still occasionally find Radeon HD 7750 cards available in a low profile form factor, and since the R7 250 "XE", with the full 640SP Cape Verde GPU, appears to be a Japan exclusive product, this remains the best best for performance from AMD in this form factor. The 7750 uses the same version of Cape Verde as the R7 250E, with 512 SPs. It's got a little more oomph than the R7 250, but not as much as you'd think: the 250's Oland GPU has higher clock speeds that keeps performance very similar, benchmarks here.
If you're after one of these there are a few that still show up on the market occasionally, though the R7 250s are much easier to find. As such, I can't really recommend any one of these - last time I checked, the two models below weren't shown anywhere, and the ones that were are no longer listed on the usual shops.
HIS iCooler H775FN1G Radeon HD 7750 1GB
Newegg link ($90)
DIAMOND Radeon HD 7750 1GB - this one is interesting; it's got a mini displayport output and a DMS59 connector that gives you two DVI outputs. Dunno if that makes it worth the asking price, though.
Newegg link ($120)
Low Profile, Single Slot - NVIDIA
GeForce GT 730 GDDR5 - while these cards have a 64-bit GDDR5 interface, it's more memory bandwidth than the 128-bit DDR3 setup would provide (40GB/s vs 28GB/s). Further, those 128-bit DDR3 cards are Fermi-based, and have only 96 shaders/cores/whatever. 384 cores on the 64-bit GDDR5 versions, along with double the ROPs (8, vs 4 on the older Fermi one) will get you the best performance of all current versions of the 730.
EVGA GeForce 730 1GB GDDR5 low profile
EVGA GeForce 730 2GB GDDR5 low profile
The heatsink on these appears to extend past the usual "single slot" width, and while the pictures on their website make it seem single slot without any protruding past the bracket, Newegg's product photos show slightly differently. I e-mailed evga about the photo discrepancy and got back this, which is good enough for me:
It is designed as a single slot solution, so it should have no problems fitting in your computer. I'm not sure why the pics differ between our site and Newegg's though. Even as pictured on their site, it should fit in a single slot area according to PCIe industry standards.
Newegg link ($70)
Amazon link ($70)
Newegg link ($80)
Amazon link ($80)
PNY GeForce 730 1GB GDDR5 low profile
If, on the other hand, you prefer something with the very thin single slot heatsink/fan, PNY has you covered. PNY doesn't list it on their website currently, although it appears to be the same cooling unit as on their low profile 740 (see below), and while they did have a PDF posted that I found a couple weeks back with the details, it seems to be gone now.
Newegg link ($65)
Amazon Link ($65 from Amazon but with long delivery delay, other sellers may have it higher priced)
GeForce GT 740 - there are both GDDR5 and DDR3 variants of these cards, but the only single-slot low profile ones I've seen so far use DDR3. It's too bad, because with a 128-bit memory interface and GDDR5, this would be a preferable alternative to the 730. The GK107 used in these has the same shader count (384) but has double the texture units (32) and ROPs (16) of the GK208 in the GeForce 730.
I'm not sure where the performance would be between these two - a GK208 GT 730 with 64 bit GDDR5 (40GB/s bandwidth) and a GK107 GT 740 with 128 bit DDR3 (~ 28GB/s bandwidth) - sometimes the extra capabilities of the GPU need more bandwidth to really stretch their legs, but I couldn't say if the GT 730 is making use of all 40GB/s that it has (or if the 740 is being held back by having less bandwidth).
Low Profile, Single Slot - AMD
Radeon R7 250 - As mentioned above. Based on AMD's "Oland" GPU which launched, as near as I can tell, as the OEM only Radeon HD 8500 and 8600 cards, these GCN architecture GPUs offer the processing capabilities of the onboard GPU in the A8 Kaveri chips (with 384 stream processors), but there are several sporting 128-bit GDDR5, giving them significantly more bandwidth than a system using DDR3 can provide. They've also been around long enough that there are a few good options:
VisionTek R7 250 1GB GDDR5 (900702)
I'd be curious to see performance comparison between this guy and one of the GeForce 730s, but I unfortunately don't have $175 lying around to do a comparison myself. IMPORTANT: There seem to be two versions of this card available. One is model 900685, which they list on their website, and one is model 900702, which is the one in the Newegg link. From what I can see from the product pictures, the 702 version has a low profile bracket, while the 685 version does not.
Newegg link ($107)
Amazon link ($93)
PowerColor AXR7 250 2GBD5-4DL
This one has 4 mini displayport outputs if you want to drive that many monitors off of a low profile card; also has 2GB of GDDR5 (which I expect you'd want with four displays). Also, fellow forum member arunphilip reports that GPU-Z is showing his as a Cape Verde card, with the 512/32/16 config, no idea if that's typical of this one.
Newegg link ($140)
Amazon link ($140)
Powercolor AXR7 250 1GBD5-HLE
1GB of GDDR5, with HDMI and Dual Link DVI-D outputs (according to their website).
Newegg link (currently out of stock)
Amazon link ($120)
The expected fall GPU refreshes have begun, kind of slowly, with the Radeon R9 285 and the R7 250XE (I don't know if this even counts, but I recall seeing rumors of it and the 285 at the same time, so I'm just grouping them together here). The R7 250XE would have been a nice addition to this list and really given AMD some punching power at the top, but it seems to be exclusive to Japan and won't be easy to find anywhere else. I'm not sure what AMD has up its sleeve. I'm hoping to see tweaks applied to Bonaire (I think they'll want to get the improved UVD everywhere they can, especially the low end) and to see that scale down. If they keep SP count of the 250X (640) but add in the efficiency improvements of Tonga (which I think will be very beneficial at the low end, with 128-bit memory interface) it could be a really nice part. Especially if they can keep power draw low enough to run just off of the PCIe slot.
It's possible that after the new GeForce series launch, which could start on September 18, we'll see Maxwell trickle down to lower power parts. Some leaked benchmarks (which, uh, might be real? Or might not?) show the top end "980" card pulling 20% higher than a 780Ti in some tests, and various posts which, as near as I can tell, all add up to "wishful thinking" and don't have actual sources all have the price being much lower than the 780Ti. That'd be great if you're the kind of person who has half a grand to spend on a video card, but I have no idea what it means for the lower end. My "wishful thinking": GeForce GTX 750Ti performance in a low profile, single slot form factor, with a power draw that doesn't require a whiny fan all day long.
In the meantime, I hope this is useful, and if anyone sees options I might have missed, better deals, better performing cards, etc, just post and I'll try to keep the list fairly up-to-date.