^ Couple of months ? Probably after January ? If Yes, then it would be a good idea to wait before deciding on the parts as Intel is going to launch their new SandyBridge CPUs, that would replace the current i5s in the same price range...
Also Nvidia and AMD are going to lauch their improved graphic cards in a months time, so keep your options open,...
As for the case, even mid towers like HAF 922 offer lot of space and airflow, so first select the best parts to go inside the case and then decide on the case...