I'm assumeing your a non-smoker and home if fairly dust free.
Pick a case with large fans with removable filters that can be cleaned and reinstalled easily.
This is what I had a couple rigs back.
http://www.amazon.com/Cooler-Master-...6544859&sr=1-1
Look at the pics get an idea of all the features.
Better link:
http://www.coolermaster-usa.com/prod...roduct_id=2870