I've always built my own PCs, but I may go for a pre-fab next time. There are a few places that will let you spec every little thing, and still bring the machine in under the cost of buying the parts. How they do it, I don't know, but I'll probably take advantage of it. Check out the level of detail at the dippily named I Buy Power and tell me that you're going to wind up with junk parts.

I know two people who bought from this site, and they're both mightily pleased.