The obvious first question would be how are you attempting to connect the mini-DP to your screen(s)? Mini-DP to mini-DP, or are you using an adapter to change it to DVI or something else? If you're using an adapter, is it an active one or a passive one?
I was trying to go from Mini-DP to DVI, HD resolution, only trying to do three monitors, since two are the same and one is not, Eyefinity would have been impossible. I ended up putting my GTX 260 back in and adding a cheap dual DVI card and was able to get an extended/shared desktop to all three monitors. Cheaper and easier than SLI I think.