Personally, I'd recommend getting a whole new case. I tried installing a new motherboard into an old Compaq case and ran into problems when it came time to connect the front USB and audio connections (proprietary pin-outs/connectors).
Things I look at when looking for a case:
1) Number of cooling fan vents. I almost always purchase replacement fans for the pre-installed fans for better cooling/quiet operations. At a minimum, I want a front intake and rear exhaust vent. Personally, I like Scythe fans.
2) Number of Hard Drive bays. For a gaming rig, I'd look for a case that has at least three hard drive bays (for a max 2 hard drives) so I can maintain a space between the OS drive and the data drive.
3) Space. My primary HTPC is a cabling nightmare. There just isn't a good place for proper cable management (air flow issues) in this micro-ATX HTPC case. It's also very difficult to remove (for cleaning/dusting/upgrading) individual components without having to remove one or two other components.
4) Construction materials. I'm not overly concerned about this as the only time I move my systems is when it's time to clean them. Otherwise, they just sit where they are. If you have a need to move your system about, you may want to look at more sturdy construction materials.
Hope this helps out
-Wolf sends