I own 5 comps ...built 2 of them myself. Two others were fairly basic, so I upgraded most of the parts myself. They're not the latest & greatest by todays standards ...but they serve their purposes
My Vista Ultimate machine I had built for me, to my specifications. I did'nt want to screw anything up on that one, as it's the primary machine I use the most. With my specs, after the build, my Vista rating came out as 5.3 ...another couple of gigs of quality ram and that rating would go pretty close to maximum.
Oddly enough, the two XP machines I built myself are the most stable of the 3 XP ones I have ... 
I've dedicated one to moderation purposes for Wincustomize. It dual-boots with Ubuntu. One is for documents/office type stuff, one has Windows Homeserver on it, another is my music/games rig ...then of course there's my baby [Vista machine] ...nothing gets on that one till I've thoroughly tested it & read all documentation. 
I still have yet another half-built computer in my closet ...I'll get around to finishing that one sometime soon, and give it to my neice.
I think I'll call it quits for a while when thats done ...I think I may have an addiction.