Erm why not build ONTO it. Frankly you vid card is good. HD is too. If you really wanna go top of the line you could afford a geforce 6800 unltra with 1000 bucks, get a good CPU for about 400 or so and get a 100 dollar motherboard. I'd go with this.
Also when you get all your parts installed. You could sell your geforce 6600 GT get some money put it towards a nice sound system or w/e esle you wanna buy. And that buy package will cost ya under 1000