It depends on the requirements of the software you run. Some newer software will not run on a 32-bit OS. But most 32-bit software will run on a 64-bit system. With Windows Ultimate 64-bit, you can even install 32-bit virtual PC running in XP mode (if your particular processor supports virtualization) to run any software that requires 32-bit on Windows XP and won't run on a 64-bit OS.
I'd go 64-bit unless you have specific hardware or software requiring you to stay in the world of 32-bit.