If your going with xp then get the 7k nvidia series or the ati equalivant. I have both Vista and XP with a 8800 GTX and some games do not run well with vista. ike 70 percent give or take have color problems and stuttoring, Half life 2 for an example does run on vista but for some odd reason after a week or so it fails to even load up. This is like after 4 vista reinstalls, Xp it works like a charm. Need for speed carbon also only gets like 20 fps with a gtx under vista. Under xp I get well over 60 fps.
On the side note, excluding games. Vista is way more stable then xp. I haven't had any freezes or anything.
Also, I would get the 6600 conroe, I wouldn't even bother ocing it neither. Cause I have one, and when running games mention above it uses just about 70-80% of it at stock. And thats with it at with 16x/16 antialias etc highest possible and i get about 90 fps in halflife 2, But the AMD you chose with the extra ghz will be equal to the 6600 game wise i bet.