64 will let you use 4 or more gig of ram. That is the main advantage. There is really no disadvantage for gamers at this time. Vista 64 is stable and the driver support is very good. Almost all software runs. There a re a few exceptions here and there but by and large this will apply only to very old and/or obscure programs. 32 bit software simply runs in 32 bit mode just as it does on a native 32 bit os. There is a wave of Vista 64 popularity right now so it looks like support for it is going to be very strong going forward. XP 64 is fine too but it never got this kind of popularity or support.
As far as XP vs. Vista: most all of the early Vista issues are fixed, despite what the Apple ads and the MS haters say, gaming benchmarks have equalized (+/-, depending on the benchmarking) and if there is any 'slowness' on Vista compared to XP it is not apparent on the kind of machines that gamers typically use.
XP wins out on absolute compatibility but Vista isn't far behind. With Vista you get native dx-10 and you are buying into the future.
My advice is to get Vista 64 and load up on ram. It's not that Vista needs so much ram, two gig is plenty, but Vista 64 can still put a lot of ram to good use with superfetch. I run 8 gig myself.
I know basic lacks the beautiful Aero interface with the transparent screens and all that which make Vista very pretty to look at. These Aero capabilities run from the graphics card and thus do not place a burden on general system performance. Basic also lacks some other things but the details escape me.. Nobody gets basic, except on very low end, bargain machines.
The real choice is between ultimate and home premium or even business. If you don't need Media Center then business is a very good choice as it has most of what Ultimate has otherwise over home premium. Ultimate has a few luxuries such as the animated desktop wallpaper that only ultimate has.
I'm not an expert on the differences but you can't go wrong with home premium really. If you don't mind the extra $$ than ultimate is nice but it really has no major functional advantage - just a few extras.
Basic is for low powered computers and provides only basic functionality. Home Premium adds the new interface, more efficient power management, the ability to use a second screen, media center and movie maker, file backup...etc...
If you use 3rd party apps for media content and DVD burning, then it may be worth your while to look at Vista Business instead of Home Premium - That has Remote Desktop Access, Full Backup and Restore functionality, and Fax capability instead of the media stuff. By going that route, you can achieve most of the functionality of Ultimate.
I would like to add that its pretty important that hardware wise your computer is Vista ready, it will make all the difference in your Vista experience.
Personally I reccommend Vista Home Premium 64, and my gaming experience has been fantastic, its hard to explain the visual difference that you see with DX10, but its more crisp and clear to the eye, but thats just a personal opinion OK!
However benchmark wise in my tests, Vista has out performed XP Pro on every test I've run, and I was dual booting Vista and XP on the same machine to run the tests, but now I'm just running Vista period because my questions have been answered.
I debated long and hard over the differences between Vista Ultimate and Home Premium and just couldn't see spending the extra money for Ultimate seeing as how motherboard wise you're limited to how much memory you can run anyway, and the small extras didn't validate the price difference to me, but thats my personal opinion too!
I don't know the type of games you play but Crysis is awesome in Vista.