For 99% of home users, "regular" Vista (32-bit) is what you should get.
Very few programs are written with 64-bit processing in mind, and you will likely experience more bugs when trying to get older programs to run in a 64-bit environment.
The only reason (in my humble opinion) to get the 64-bit version is if you are a specific type of computer or game programmer, or if you just enjoy fixing inevitable problems and incompatibilities that occur with any version of Windows.
3 to 5 years from now (2011+) we will probably start to see a lot more 64-bit applications, but for the home user or casual gamer, I would avoid 64-bit like the plague, as it is still not 100% "mature". Don't be an early adopter unless you have a specific reason.