I used to be an Nvidia devotee but they totally fucked over a large number of users by not making or supporting drivers on a ton of Vista-gen cards. I know that Vista was a hardware-software nightmare for everyone, but Nvidia has some shady dealings with companies like Dell and their driver support is so-so to the point that the mod community has supplemented a ton of Nvidia drivers.
I can't speak about ATI's evils since I'm not as experienced, but I have heard good things about their cards and I'm sure you are fine either way.
@Prof S: If you go here:
http://www.newegg.com/Store/SubCateg...0&Order=RATING
For not a lot of beans you can get a pretty sweet pre-built machine these days. Obviously you have more control over the finer details if you build yourself, but if there has ever been a time in history where buying pre-built is almost as good as building yourself, it would be today.
The reality is that if you buy a manufactured PC you are going to see _SOME_ corners cut. So go with the reviews and the price and the spces you want. The PC gaming video curve seems to have hit a bit of a lull, so any card made within the past 2 or 3 years should tackle any recent game. If you want to play the killer game coming out in 2 years you'll want a top of the line GPU set up.
My advice is to chop 150 off your PC price and invest in buying Windows...this way you can just reformat the PC from the get-go and wipe out all the crap and bloatware that is going to come installed.