Help Me Choose a Gaming PC Case!

Welcome to Tom's Hardware Forums,
Very nice, what system do you have to place those into?
If a pre-built I'm going to venture a new power supply would be in order along with a case

You're going to want at least a 600W (good quality) PSU if you want to use both cards. They will require a PSU with 2 x 6 pin and 2 x 8 pin PCIe connectors (unless you use the inclyded adapters to make regular Molex connectors work). You will also need a motherboard that can do CrossfireX. Also, they quite long cards. They won't fit in a small case.