consider itx, or certainly micro atx. Mini itx would be betterhttp://www.overclockers.co.uk/showproduct.php?prodid=CA-073-BX
Someone on here has pics of a build. i'm seriously considering it.
There are smaller, overheating is related to how many changes of air you get per minute, i.e. the ration of fan cfm to space in the case. Imagine a really big case with tiny fans vs a reall small case with big fans, which has the most fresh air for cooling?