Gotta go with Rondeemc- You can only go just so small without having to sellout on the psu. That's no big deal for a net-surfing/word processing box having a 700 celeron, works great. Add 1800+XP and power hungry vid card, fans, extra drives, etc.- you've got a problem.
The Fong Kai he mentions is easily the best buy I've seen in the category- there are other suitable cases, but they come with wimpy psu's that you have to change out, many have poor ventilation and need mods- The lite0n fs050 and the 500 series superpowers just need bigger psu's, the Inwin v500 needs that and major cooling mods, too.... and they're all about the same size as the Fong Kai....