Indeed, for the majority of GAF, the i5 2500k has 90% of the performance of the i7 2600k in games (And that's before you overclock), yet is $100/£60 cheaper.
Personally, I have just upgraded from a Q9400 (Clocked at a handy 3.6Ghz), so will post specs below, needless to say the upgrade is bigger than even I expected. Games like F1 2010 have been transformed as I finally get 60+ FPS at max settings using 1920 x 1080. The thing is just as cool as my Q9400 was (So ~58degrees under IBT load, ~50degrees in games) at 4.5Ghz using 1.31Vcore. Any higher requires stupidly high Vcore so I think the chip has issues above it.
My 2011 rig:
Intel i5 2500k @ 4500Mhz (Cooled by my old, yet capable Zalman CNPS 9500A, Frio to come when Spring hits)
ASRock P67 Extreme4 (Bought for £115, supports SLI/Tri-Crossfire, a bargain for the money)
4GB Corsair XMS3 (Using 9-9-9-24 Timings, 1.5v)
XFX Radeon HD5870 (Clocked at 970Mhz/5000Mhz, 1.3v)
So £350 (For CPU/Motherboard/RAM) for my rig to enter the next decade, I'm dead chuffed.