Two monitors, one box, which way is best?

I am currently just trying to dualbox but in the future I may expand on that.

For now though, I have a dual-core rig that's about a year old with an 8800 GTS in it, in a SLI-capable motherboard but without using SLI.

There are three ways I can hook up a second monitor. The 8800 has two video-outs. The first way is to set it up as a dual-monitor emulation, just like I had added a second video card. This allows them to be different resolutions, and for me to position them so they line up just as they do in real life. Problem is, that in WoW the graphics run rather slow. I have tried, just for fun, running 4 instances of WoW on a single monitor, and those ran better than running two in the dual-view configuration.

The second way is to add another video card. Since I have a SLI-capable motherboard, I have two full length slots to install video cards. I tried adding another card (a 9600), and it seemed to work OK at the start, but within 5 minutes of running one WoW on each card the framerate tanked. To like 2 FPS. Tried rebooting, driver updates, nothing fixed it. Eventually I gave up and installed the 9600 in a different computer.

The last way I haven't tried yet but it may be my only option. I could set the video card to be in "span"mode, so that Windows thinks I have one double-wide monitor rather than two small ones. This is less than ideal as my main monitor is over 50% bigger than my secondary one, and in order to do this the resolutions need to be the same on both. Given that the computer performs OK doing 4x on one monitor, I can guess that this would work fairly well, but would be rather inconvenient for normal use, as Windows will open browser windows spread across both screens and such.

There's a small wrinkle in that when I installed the 9600 in my second computer, that computer has had some weird glitches. I can't be sure that it's the video card's fault, but it seems like there's a fair chance I got a bad card. So I was wondering if anyone else had installed 2 video cards on a SLI-capable motherboard and had 2x (or more) instances of WoW working fine. Or if anyone had any other solutions I could try.

Thanks,

Sethra

(I'm in Syro's guild btw, he posts around here as well).