Depends on how much you want to spend on it? and is it a whole new system, or just a processor?
And what sort of gaming are you going to be doing?
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Most would probably suggest an AMD Processor.
A Phenom II x 4 (Quad core), 955 black edition, with a little overclock, is quite favourable at the moment, and will handle a fair amount of older and current games on MAX or high Settings, giving good results.