64 bit Vista will run all 32 bit Vista programs. The main thing that you need to make sure of is that you have 64 bit versions for all of your device drivers. The 32 bit versions will not work in 64 bit Vista. Plus, of course you will need a 64 bit CPU.
If you have all this, then I would say go for the 64 bit Vista. It runs faster and supports more RAM.
Hope this helps in your decision,
Shawn