Galaxy and KFA2 have announced a low-profile GeForce GTX750Ti.
That half-height 750Ti has a dual slot heatsink...so it won't fit in the ISK300-150. I completely agree that a 7750 is as low as I'd go for a gaming dGPU.
Just wanted to point out that I'm putting together a system in an ISK300-150 right now and have been keeping an eye on video cards that will actually fit in the case. VisionTek has a 1GB GDDR5 R7 250 card
that will fit.
There's also a Diamond 1GB GDDR5 R7 240
that might fit - might. It isn't double wide, but it sticks out a bit. The shroud might end up pressing against the honeycomb grill in the side of the case. Not sure about that, but if it fits, it'd probably be quieter than the R7 250.
I don't see any GeForce GT 640 cards with GDDR5 that will fit - the Zotac ones are too wide and the Asus one doesn't include a low profile bracket (which, uh, doesn't make much sense).
I'm hoping to see a GeForce GT 740 in low profile form factor that will work, maybe a mid-cycle maxwell refresh? Or maybe someone will decide it's worth it to do a low profile 750 with a single slot, blower style shrouded cooler? Well, probably not, but I can dream.