Although programs and stuff made specifically for 64-bit applications havent really come out yet (and they probably won't for a while because there simply isnt enough of a customer base using them right now), 64-bits are known to perform much better when it comes to gaming.
However, I hear that normal 32-bits sometimes perform better when it comes to programming, encoding, and graphic programs.
For more information, you might want to read up a bit at
www.anandtech.com or
www.tomshardware.com