The resolution isn't the major factor. It's the number of polygons being rendered, lighting, shadows, bloom, particles, and other effects that will determine what card and or sli/crossfire support.
As well, a monitor with 60hz refresh should ideally have a card that can provide 60fps consistently before it becomes overkill. Since the OP mentioned that without SLI he was getting 40-60 FPS, that s
Another trick we tried a few weeks back was to set the custom program settings to Force Alternate Frame Rendering 2. Seemed to make SLI work back then.
In terms of single monitors, you have overkill. There are known issues,(wither ZO will admit or not) in towns and areas that has large amounts of players. Watch your GPU usage and clock rate in the ar