Well man, if you don't like Vista for whatever reasons, then your only choices are XP and XP64. Surely you've been around the boards long enough to have seen the threads concerning 4GB of RAM and 32bit XP or Vista.
If you haven't, the simplified version is 4 gigs of ram won't show up as 4 gigs in xp or vista (but vista sp1 changes that cosmetically only). So less than 4 gigs of RAM you will be fine with 32bit, 4 or more and you want 64bit.
Now if you want more than 4 and want to game you've just backed yourself into a corner by not liking Vista, because XP64 has pretty poor driver support. XP64 may still work fine but you'll need to know exactly what hardware you plan on using with it and whether there are 64 bit drivers for it.
My recommendation: Vista 64.