This would be my recommendation. I have one loaded with 4 nVidia cards (different project) and it works perfectly.http://blog.cryptohaze.com/2011/06/quad-gpu-builds-with-case.html
is my build - I wanted a case, since I like moving things around. The Antec 900v3 (and apparently v2) have 8 expansion slots in the back to support 4 dual-height GPUs.
You can certainly screw around with extenders, expanders, etc, but personally, I don't think it's worth it. Then again, I did build this for other projects, so cost was less of an issue (I didn't build it to do bitcoin mining - clearly).