Don't listen to
>>2! AMD chips will run any modern game, unless you have something ancient like a K4-K6 chip. 64 bit does offer greater performance, even without a 64-bit OS, which WILL also get MORE performance (depending on what you do with it, it may or may not be noticeable compared to a 32-bit OS). AMD's socket 939 Opterons are currently the best thing out there by any measure. They are based on 5 year old technology, but still surpass all intels (even the .65nm chips) in nearly every way. Intels CURRENT .65nm chips will get you the highest overclocks EVER. The Presler core is capable of 6GHz. That being said, AMD will be soon (in the next few months be announcing AM-2, and socket 939 will BE OBSOLETED. Also, Intel will also this year be shipping their Conroe core CPU's which they alledge will outperform EVERYTHING. If you want to wait, it could be worth it. If you want to buy now, I recommend
AMD: Opteron, or San Diego/Toledo type A64.
Intel: Yonah or Presler (Yonahs are used in the new Macs, dual core, .65nm, no 64bit support, Presler are dualcore, 64bit with EM64T instructionset, but don't perform as well as the A64 X2 CPUs)