Don't build.. 'It's so much cheaper! You can have a better computer!!' bullshit..and it'll take you forever, and cost you a lot of time and work, and things will go wrong.
www.digitalstormonline.com
Probably one of the best sites to get it from, their prices aren't that bad at all, and they've got options out the ass. Even their lowest-end performance PC is badass. They set up, overclock, and stress test your machine for 3 straight days before sending it to you. Much less of a headache..
But building it will be a little cheaper.