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Unless you're having more or equal to 4 gigs of RAM, there is no real need for a 64 bit OS.
If you've got a 32 bit OS, and you're installing 4 gigs of RAM, then only 3.25 or so gigs are usable.
You won't see any better gaming performance. However, if you have 4GB or more of RAM then your better to use 64-bit. You will be able to open and run more programs simultaniously before you run out of memory.