You only have two Versions (as far as I know), Vista 32-bit or Vista 64-bit. The 64-bit is okay so long as all your applications support it. Stick to the 32-bit version. Personally, I would stick with XP, but that's just me.
If you're going to go for the 64-bit vwesion, make sure you have drivers for all your hardware, including peripherals. Also, some software, those with 16-bit installers, wont install on the 64-bit version of Vista.
I've just bought a new PC with Vista installed, but I reformatted the disks and installed my old XP because the first half dozen hardware and software I tried to install from my old PC came up with "known compatibility issues" messages.
So, if you plan to continue using your old software and hardware (broadband modem was my problem), best to check that it's Vista compatible (or vice versa) before you take the plunge.