I'm not sure, I have a single monitor setup. Here's the thing. Multi monitor setups have a much higher resolution, which taxes the VRam. What takes up even MORE of that Vram? AA. I'm sure it depends from game to game but AA is best left off on taxing games. Newer games like BF3 can use the MUCH less taxing FXAA. If the game has that, use it. I think it 2x AA will be alright. Maybe 4x. Just derp around with the settings until you get a good mix.
Keep in mind, Source does not support SLI (I think), so you'll basically be running off a single GTX 460. Things could turn into a slideshow.
Guru3D uses the following games in their test suite, COD-MW, Bad Company 2, Dirt 2, Far Cry 2, Metro 2033, Dawn of Discovery, Crysis Warhead. Total fps (summing fps in each game @ 1920 x 1200) for the various options in parenthesis (single card / SL or CF) are tabulated below along with their cost in dollars per frame single card - CF or SLI:
So for $125, ya gonna get 592 fps..... compare that with .....
$450 for a 580 and 616 fps .... 4% more performance for 3.6 times the price
$480 for a 7950 and 603 fps .... 2% more performance for 3.8 times the price
$560 for a 7970 and 675 fps .... 14% more performance for 4.5 times the price
Nah Surround will work, you just might not get the extra performance of the second card. It could be that the second card will essentially be a pipeline, as opposed to a double in performance. This would be specific to HL2/Source games/any game not supporting SLI. Most modern titles support SLI.