What's your budget?

XP > Vista if you are looking at it from a gaming viewpoint. I will post supporting info if necessary, but take my word for it.

If you do get the dell, get the 8600GTS, as it is a little bit better than the 7900GS plus you will get DX10 support.

If I were you I would just build it yourself, as it's very simple to do. If you are interested I can post a parts list based on your budget.

-BA