75% of all people who use computers could build one themselves. It's extremly simple. I could stand there and tell you what to do and anyone could do it.
In addition to Mem Exp.
www.ncix.com is a pretty decent place also. They have a computer builder on their site, so you can custom pick your parts, and if you want they'll put them together for you, or send them to you so you can build it. If you compare though, Mem Exp has some better prices.