It really just depends on what you play. It's impossible to say XYZ CPU is a bottleneck for ABC GPU globally, because sometimes the CPU just doesn't really matter, and other times you'll be CPU-limited no matter what you have.
Case #1: RAGE -- RAGE does not really care what CPU you have, as long as your GPU is fast enough to do the virtual texture decoding. I've seen RAGE played flawlessly at 1440p 60FPS on a Core 2 Duo (stock speed!) and a GTX 580.
Case #2: TERA Online -- TERA does not really care what GPU you have, as long as it's past a certain point of performance. Some friends of mine saw performance improve a lot going from Intel HD 3000 to various Fermi-based Geforce cards, but I saw minimal performance improvement going from SLI 460s to a GTX Titan, so I tested CPUs briefly -- it runs about the same on a Core i3-3220 as it does on my 3570K, but going from 3.4ghz -> 4.4Ghz on my Core i5 made a bit of difference (not much!), and it still bogs pretty hard when you get into it.
As an aside, most MMORPGs exhibit similar performance characteristics as the above, including Everquest II, World of Warcraft, Champions Online, Neverwinter, The Old Republic, Guild Wars 2, and so on.
Case #3: Warframe -- Warframe does not really care what CPU or GPU you have, because it has both 32-bit and 64-bit versions of its DX9 and DX11 renderers, and it's going to run like bollocks in any case. I've seen Warframe played on tons of machines (it's free, after all), and your maximum framerate can be quite high if you have fast hardware, but your minimum framerate -- and average, over a long period -- is going to be no higher than the 30-40 range no matter what you have. I'd like to chalk this one up to the game's "Beta" nature, but it's been "Beta" for over 2 years now and nothing's really changed on this front -- if anything, it's gotten *worse*. The netcode sure has.
i5-3570K @ 4.4 (NH-C14), 4x8GB DDR3-1866, GA-Z68MA-D3H-B2, ASUS GTXTITAN-6GD5, 128GB Vertex 4 / 2x60GB Vertex Plus R2 / 2x2TB Barracuda 7200.14 RAID0 / ANS-9010 (4x4GB), SST-DA1000 (PSU), 2x VS229H-P, 1x VG248QE, 1x MIMO 720F, Corsair Vengeance K90+M95