SLI cannot do 4, 3 is the max. Eyefinity can do 4. I am running 3, 23" Dell SP2309W's. Total resolution of 6484 x 1152 off of 2 580's in Surround, SLI enabled. I can play all but the most demanding games at their highest settings with 30 - 60+ fps, depending on the game. For the most part, running any AA with that res is just too much for it but, for some games you can.
BF3, Ultra settings
Batman, High settings, lots of Physx going on.
Any COD on the highest settings
GTA IV all the way up. All sliders maxed
Metro 2033, very high, no AA
From what I have been told (by EVGA tech) is that if I want to run my games at this resolution with the same settings as I did in a single monitor set up, I need more VRAM. Go for 3gb cards and you should be all good. As of now I need to tone it down because of the 1.5gb limitation of my current cards.
Hope that helps. ;)
<message edited by JimPapp on Wednesday, February 29, 2012 3:35 AM>