Hmm, for me function always defined form.... never the other way around.
I run anywhere from 6 to 10 computers at any given time. One is a desktop with 2 video cards in it... that prolly cost more in total then the rest of my PC's put together. Thats what my games get played on. Some are laptops (mostly for servers of some or other type), some are "book size" systems (more servers, or media center), and the rest are Shuttle branded PC's - about the size of a large toaster. (One is used as a firewall/Gateway, and a few more as "family PC's")
I guess what I am really trying to say is what you build needs to be be defined by what you intend to use it for. 400 bucks will get you a top-of-the-line Shuttle box and at least 8 gigs of decent ram for it. The Case-CPU-Mobo are all custom and come assembled out of the box - you just add your own ram, cpu, and any expansion cards... if needed.