If I can throw my two cents in as well,
I've been researching micro ATX gaming rigs and performance rigs for the past two days and I'm actually really impressed with what I'm seeing. Just search around for "micro ATX gaming" and other searches similar to that... for example thermaltake I believe makes a case called the lanbox.. such small cases were intended for portable gaming. So maybe you'll be able to find some answers within that research.
The past few hours... (and yes I know it's 5:30 AM..in NY at least..) I think I've come to settle with the Thermaltake A30 microatx case. http://thermaltakeusa.com/Product.aspx?C=1466&ID=1989
maybe you'd be interested in something like that?
either way, it can be done.
Check out Silverstone and In Win for HTPC micro atx cases too.