I will give a quick EZ answer then one for you to ponder.
I5 2500 is currently your best choice for gaming. Games in general don't use a lit of cores. The i5 is best.
Now the one to ponder
In the next couple of years I would imagine games will utilize more cores. Intel is best in the per core market right now. AMD is not. Based on price and performance AMD is better in the multicore market. That being said I am going to build an FX setup to help "future proof" myself. Yes bad term to use, I know. Windows 8 will help AMD due to core assignment issues right now. And looking at the results in benchmarks I am fine with AMD VS I5 2500.
think about it this way. Example i5 plays a game at 100 fps the 8150/8120 plays it at 80 fps.
Look at both games and you will see no difference. That's my view. The numbers are real. I5 2500k kicks the AMD FX 8150/8120 in the balls on gaming. But at such a high fps rate. You won't be able to tell.
As for future proofing for games (my opinion) the more cores/threads the better. So I'm my eyes it is a i7 2600k (4 cores + 4 threads) vs 8150/8120 (4 modules = 8 cores) comparison on the build. AMD is less expensive with those comparisons and with Windows 8 coming out which will help with the current bulldozer chips and more so the piledriver (we hope). It only makes sense to me to go AMD.
Core assignment is
1,2,3,4,5,6,7,8
But should be
1,3,5,7,2,4,6,8
For info on the core assignment issue check out the other Toms forums or Google it. Basically its like hyper threading gone wrong in a nut shell. Or you can wait on ivy bridge to come out then compare.
So much to think about. Blah!
I have gone on long enough. Let the yelling and arguing begin.