Then your options are many! There arn't any AM3+ form factors out besides ATX right now, so there are good cases and MBs that can be had for relatively cheap.
techreport.com and anandtech.com have great reviews on cases, browse around on newegg to see what you want and I'll help you decide.
I'm going to go with a fractal design case most likely when I build my new rig. I want to go mATX ( with the Fractal Design Arc Mini), but that depends on BD performance and the boards the MB manufacturers release after launch. If BD is worth it, i'll stick with AMD and the MBs are there, if not I'm going with a 2500k and a maximus IV gene-z for the form factor.