Such systems are called barebones computers, and if you do a Google search on these, you’ll find a lot of comentary about them.
I got the Pundit-R case by itself, without any other components. Just for grins, I plugged in the power supply, and found that the thing sounds like you are standing next to a jet engine. Absolutely the loudest thing in the room. However, there is supposed to be a way to throttle the fan back, so I’ll I will reserve judgement until the system is built.
I ordered a Pentium 4 processor, a megabyte of RAM, a CD-ROM and a hard drive for the unit. We’ll have to see how it all goes together.
The most popular SFF boxes today from Shuttle, MSI, and Biostar are all of similar size, having roughly square dimensions in height and width and being somewhat longer. This fits a small power supply of 200 to 250 watts and gives enough room for a 60mm or 80mm fan in back for system and CPU cooling. A back panel full of VGA, USB, ethernet, sound, and firewire connectors provide a complete set of expansion opportunities. The front panels usually have USB, Firewire, and audio ports for additional expansion. Custom or semi-custom CPU heatsink/fan combinations are provided by most makers, with the most notable being Shuttle’s heat-pipe with an 80mm exhaust fan. Two expansion slots, usually one AGP and one PCI, makes it into most SFF systems. Vented side panels help keep high-end video cards cool, and the expansion slots usually have room for full height cards so that the latest high-end video cards will fit.
I’m building a reference page here.