DON'T BUY A PREBUILT DESKTOP
Custom built may be the best way to go, but I think it's a bit unreasonable to expect everybody to have such skills. Not everybody is skilled enough to build their own computer.
you'd also be if you bought the 512MB 8800GT because the 1GB GT is a way to prey on consumers that don't know anything about VRAM and just think MORE MEGABYTEEEES = more power
In the case of games, the high resolution textures with the latest games are pushing memory through the roof. More VRAM will help most of the newest games.
ATi not only has the advantage at the moment but you also get better picture quality and far better driver development.
There is no picture quality difference between the manufacturers for games. A high end nVidia card will give you the same results as a high end ATI card.
Driver quality is the same right now. Both manufacturers have very stable drivers. I have zero troubles with games with my nVidia video card with current drivers.
At the moment the only people buying nVidia cards are the people looking for THE top card, so far thats the 9800 GX2 which costs roughly $500 USD.
I think nVidia is trying to push their GeForce GTX 280s as the latest video card. By the way, Newegg is selling GX2s for
14133231">$300.