SP1 will force 32bit Vista to show all 4GB, but it still can't address it all. I have 64 bit and I've not had a single issue. Just make your wireless adapter etc have drivers for 64 bit before buying it although they almost definitely do. And about XP vs Vista. I used to have XP on my old PC before I built my current rig so I can't comment on performance, but you could read this article: http://www.tomshardware.com/2007/01/29/xp-vs-vista/
Obviously, that article is quite old, but it's true that you will see a slight performance decrease in games with Vista compared to XP, but XP can't do DX10 and there will be a cut off date where a few games will stop supporting XP.
I game occasionally and built a rig with an Intel Quad Core, 8 Gigs of DDR2 in Dual Channel, 2 8800 Ultras in SLI, and 1TB of storage with Viista Ultimate running off of a Raptor.
And you game occassionaly!? Wow!