I downloaded and ran the Windows 7 Upgrade Advisor and the 64-bit report looks good.
There are a couple programs that aren't compatible but they are MS stuff that would probably not be reinstalled anyway after the upgrade.
The only other notices are about updating drivers on printers and stuff.
I know I'll have to update stuff for the BIOS (?) and chipset and that's the kind of stuff that worries me - hardware that I installed or added, I know about and can tick off on a list as I go. The innards that came with the rig are a different matter.
Anyway, looking at Win7 it appears that if you buy it you get both 32 and 64 bit versions so if I upgraded the OS at all, I'd still have either option to fall back on.
Yikes, though - I've got a 1TB external drive that all my music and backups and program setup files are on. Will that have to be reformatted if it's not a bootable drive in order to access the files again? I'd have to back up 200+ gigs of data somewhere.