No. 4

Lets pretend you are dual-boxing.

Say you have 2 gfx and a monitor hanging off each gfx. One monitor is always the primary monitor.

You put a WoW instance on the primary monitor and a WoW instance on the secondary monitor.

If you do this your WoW performance will be WORSE than if you only had one gfx card and 2 monitors. This is because WoW only renders to the primary gfx. The driver software has to copy the image from the primary gfx to the secondary gfx causing a lot of slowdown.

You have two options that I know of:

1) Run 1 gfx and 2 monitors.
2) Run 2 gfx and 2 monitors and use InnerSpace by Lax which makes WoW render on the correct gfx card. Disclaimer: I don't use InnerSpace and I only read that InnerSpace does this. Can someone confirm?