SFF, or small form factor, is what is behind those tiny, cube-like PCs that can be carried around very, very easily. They don't hold much - no RAID arrays or massive graphics cards or multiple DVD writers - but then you don't go with SFF for that. And besides, with the kinds of capacities we are seeing in HDDs and RAM recently, limiting a PC to a single HDD or having a mobo with only 2 DIMM slots isn't potentially the crime it used to be.

Extremetech (http://www.extremetech.com) have an article on a small form factor roundup (http://www.extremetech.com/article2/0,1558,1645837,00.asp?kc=ETRSS02129TX1K0000532), where they discuss some of the SFF kit that is around, and what's possible with it. There's plenty here to guide you into building that LAN party cube.