32 bit software should "just work" due to the fact that 64 bit windows has a thing called WoW (Windows on windows) which is a compatibility layer which allows it to run 32 bit apps (32 bit versions of windows have the same thing to allow it to run 16 bit apps)
Which version of windows are you using?
"XP Pro" x64? Or vista x64??
XP Pro 64 is crap with regards to app/driver support. It wont run hardly anything because its not xp under the hood (its based on windows server 2003). Vista 64 bit on the other hand, normally tends to run 99% of the stuff that vista 32 bit does.
I don't agree that XP Pro 64 is crap. Sure there's poorly written (but I concede useful) stuff that won't work in XP-64; it's as often as not the installer that's the problem, I've found. As for drivers, I've got stacks of hardware attached and 64 bit drivers are readuily available.
jbennet is right about Vista 54 vs Vista 62; but then Vista doesn't run eveything that XP can run without buying new software versions.
Basically Windows as a whole is a sorry mess in terms of application inter-operability.
Finally, a word about choosing a 64 bit OS; you do need to think why you'd go there I wanted improved floating point performance for 64 bit applications. Others wanted to use the larger memory space to provide for standard applications concurrency without excessive roll-out/roll-in.