Right, some points about dual-GPU:
Good performance in one game doesn't gaurantee it in another. Some games work well with SLI/Crossfire, some don't. New games can not work at all (only use 1 GPU) untill the next driver update.
4GB memory means you have 2GB effectively. Each GPU uses it's own 2GB, with data cloned on the 2 sets. (not that you will fall short with 2GB, but it should be noted)
Oh, and the GTX590 beats everything hands down.
(unless you care about the power bill)