I've got 2 5970's on hand (lucky me). Would something like this rig work just swapping the 5850s for 5970s?
Mostly the problem is that PSU, as well as cooling. With two 5970's you'll want a quality 850Watt PSU or better, with at least 70amps on the +12V rails. If you put those two in right now and loaded it you'd easily exceed the 600 watts since the GPUs alone take 300 watts a piece and your computer has other parts in it.