I'm running two monitors, off of a 1GB GTX 570.
They're 27" Samsungs, with 1920x1080 resolution.
IS Boxer renders five regions at 1920x1040 (I have the taskbar visible).
The master is on one, and the four slaves on the other.
My master is capped at 60 FPS, the slaves at 20 FPS.
The slaves are on minimal settings, except about 1/3rd for view distance.
The master is on medium-high settings, with maxed view distance.
Shadows & Weather Effects are off, across the team.
I'd think a 3GB GTX 580 will be more than adequate.