http://www.ibuypower.com/mall/lobby.htm
http://www.cyberpowerpc.com/
Both of those sites will let you configure your system to your specifications. I was able to piece together a decent AMD system with an Athlon X2 5000+ Black Edition, an Nvidia 9600GT, a decent 600W PSU and Microsoft Vista Home Premium at both sites for about $800-$850, depending on the other parts you pick. Should you want to venture into Intel, the price inflates a bit to over $900. I don't know how tight you are with your budget, but there you go.
Play around with the configurations and see what suits you best. If you have any questions on any of the parts, feel free to post them here for feedback.