Up to this point, I was not able to confirm or deny the support for more than 4 GPUs on Windows 7 64bit. However, I was able to find various users that have been able to run 5 and 6 gpus on Ubuntu 8.04 64bit.
http://forums.amd.com/forum/messageview.cfm?catid=328&threadid=122135
So, to answer my own question: If you are going to build a GPU Powerhouse utilizing >4 GPUs, currently the only documented solution involves 64bit Ubuntu.
Hope this helps someone.