What about setting up a dual boot XP/Win7 option? Then you can compare how well each does with your software.
Stick with XP/32 if you do go ahead. You might find it harder to get driver support for XP/64 on newer hardware.
I was going to suggest that as well. It's the best of both worlds. Plus, if you decide you don't like the way one of the OS' is working, you haven't wasted a ton of time reinstalling the other.