Strictly for gaming, you wouldn't need much more than a dual-core processor. Namely the AMD Phenom II X2 550 black edition. It runs at a cool 3.10 Ghz (3.8-4 Ghz overclocked) and tops most quad-cores in many games (like Crysis).
My advise would be to go for the Phenom II X3 720 BE. It has similar performance to the X2 550, plus a bit of future proofing as well.
Unless you have the cash to burn, that is. Then the expensive i7's and i9's are at your disposal :)
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