I'm running 2 rigs in mATX based SFF cases. Check the link in my signature for pictures. I love the smaller size and good looks of these cases. There's plenty of room inside for all of the componenets I need while still allowing good airflow.
I'm not a fan of dual video cards, so I've never looked for a mATX crossfire mobo. However, I'm sure there's at least one out there!