Sorry to post in an old thread but since I was considering picking up a second 5970 recently, and stumbled across this thread, I figured I'd drop my 2 cents.
As someone considering dual 5970, coming from a single 5970, I can say that it is not necessarily overkill. Sure, for people with one monitor it would be. But even a 5970 is pushed to its limits at 5760x1080 eyefinity, which is 3 monitors. I have mine clocked to 900/1200 and it is still not good enough to use Edge Detect AA in some of the harsher games. Usually when in Eyefinity mode, I am using Box AA at like 4X. So a single 5970 is by no means an end all god card capable of doing everything.