yeah, not pro vs ultimate that matters but 32 or 64 bit.
And /usually/ 64 bit works well. I've mostly not had much problem with programs or drivers... "mostly".
A lot of drivers like printer drivers are user mode so you can use 32 bit drivers. Some, like sound card and video drivers are NOT. You need 64 bit versions of those, but luckily, win7 x64 has been out long enough that most all companies now support it. Took a while for M-Audio to catch up with the drivers for my delta 44 audio card but they finally did.
If you want to be super safe and you don;t have more than 4 gig of ram anyway, install the 32 bit version. Otherwise give the 64 bit version a try, it'll likely work out well.