The happiest I've ever been with 2 GPU's was when I was playing FS2004 in multi-window mode and the 2 GPU's each have 2 monitor plug-ins.
So I was playing a flight sim with 4 monitors!
I had two in front of me, stacked one on top of the other. The top one was my windshield and the bottom held my radio deck, GPS, and various other dials. The other two, I had to my right and left giving me a view out each window. Very panoramic; it was very cool.
I think I actually liked it more than flying real planes; it wasn't loud, I didn't have to sweat, there weren't a multitude of idiots that I had to keep constantly watching for. It was fun.
Um... but of course, they were not running in SLI. Which I think people forget about a lot. You can only run SLI on a single monitor.
Running in SLI, I see some performance gains in some games; but not 2:1, more like 1.3:1 or something like that, not a huge improvement. If SH allowed multi-window play, I'd ditch SLI mode in a heartbeat to have the extra displays!
So if you're thinkin dual GPU's, think games that support lots of monitors... (did I mention that buying 4 monitors is not cheap?) or I just don't see that much bang for the buck. My last GPU purchases were dual 9600GT's for about $300. Which I didn't think was too bad at all. Oh, and big power supply, and at least a couple 120mm fans.
If you're just gonna run 1 monitor, I'd say there are probably better ways to spend your money than doubling down on a GPU.