I have 32 bit Ultimate, my room mate has 64 bit Ultimate. The only real difference so far is performance. Vista is a bit of a memory hog, but if you're equipped for it it's great. As far as everything that's in your computer, Microsoft has this nifty little thing called the Windows Experience Index that rates each of your components on a scale of 1-6 (it may be higher now, but I doubt it) and your overall score is your lowest on all of your major components, grading your system based on it's bottleneck. I suggest looking into that and seeing if you can run tests on hypothetical builds for a new computer, that might give you more insight on where you can save a few bucks and where you should invest more.

[Linked Image from zandadev.com]