I bought a retail ultimate upgrade (got a good deal) and got both 64bit and 32 bit disks. You can change from one to the other (as long as you are not running both at the same time) if your requirements change, although this means reinstallation from scratch. Otherwise, games will still be written in 32bit for quite a while yet. If you buy OEM, you have to choose between 32bit or 64bit as you do not get both disks.
Otherwise, here is a good place to start researching what works or doesn't in Vista.
Vista Software Compatibility List
FWIW, I am running mine on a MacbookPro and it runs much hotter than OS X. I found this utility invaluable on my laptop. It may of some use on an iMac as Bootcamp drivers are written for XP and not all work on Vista.
Input Remapper