I've always been a fan of CyberPower (http://www.cyberpowerpc.com/)
for those who don't want to build their own system. It allows you to customize your PC to fit your budget.
iBUYPOWER has a similar offering (http://www.ibuypower.com/)
where you can customize your PC to your hearts content.
Just do some research on parts first. This site has some great monthly graphics card reviews, and some CPU reviews