The smallest you would be able to go would be a micro ATX case, and still let you use standard size parts. There are smaller cases, but then you would limit yourself to certain parts that are made for them.
Newegg.com - COOLER MASTER Centurion Micro ATX 541 RC-541-SKR1 Black Aluminumm bezel, SECC chassis MicroATX Mini Tower Computer Case 380W Power Supply - Retail
something like this would allow for plenty of airflow and still be kinda small, while still being able to upgrade later on without much worry. And since it has a decent PSU included, you could throw in a 7600GT and be able to run most games with it.