Second newegg and mwave--mwave is a smaller company with a pretty bad web page, but they have decent prices, and if you want a full system, they'll assemble all the parts you buy from them for something like $35. Last time I built a full system, I priced them out and even with the assembly fee it was only 75 bucks more than newegg, which to me was the magic number to let them deal with all the hassle.
JJ, I had never heard of txmicro before, I'll check them out. I've nevered ordered from woot, but I like the whole idea behind what they do and the way they do it.