Don't bother with Ultimate - MSFT has delivered on very few of the promised extras.
32 or 64? I've run both: They look/feel/behave pretty much the same. From a usage standpoint, 32's defaults are more home computing friendly. 64 likes to nuke things who's security certificates it doesn't like, for example. Kind of annoying. I haven't found anything yet that out and out won't run if it gets by security. (Speedfan and Coretemp aren't liked by 64 because of their security certificates).
OTOH, in and of itself 64 runs noticably quicker. Not "OHH EMM GEE" faster, but it's definitely snappier.
Games? I have Bioshock, NWN2, and Oblivion installed and haven't had any trouble running them at all. WOW runs beautifully as well. But I can't leave that on my comp - It's Ev1L and tries to take over my life. They look a bit prettier on 64 then they did on 32, but I'm pretty sure that's mostly because nVidia have a 64 bit driver for my card. Not as much a function of the OS.
Which version? Premium gets you Media Center. But if you use different software for that, then stick with the regular Home one. Oh - I do prefer a retail license, for less hassles reinstalling if I screw something up.
Also - Hard Drives are cheap, and Dual Booting is a great way to have your cake and eat it too.