Honestly is really really easy to put a machine together...

look at

http://forums.overclockers.co.uk/

All you need to know about building a PC very helpful bunch its a UK website but i suspect that there is something similar in the US.

Putting your own PC together will mean

1. better spec for your money!!
2. everything do right
3. an achievement.

Other then that you could always go www.alienware.com