I reuse the cases. I build the new PC using one of these. http://www.highspeedpc.com/
I get the new one running with a new boot HD or SSD on the bench. Always have an extra DVD player around to hook up the the new mobo on the bench. Install all my software while system is on the bench. This is normally the longest part of the project! And this way if I get tired I still have the old working system in the case. When all is ready it is a matter of swapping out mobo and boot drive. All my data is on other physical drives that stay in the old case. In a couple of hours I am up and running with a new system in the old box.
My latest setup includes the Intel liquid cooling solution and a case with 3 6" fans. Very quiet!