Performance Gain of using 2 8800s vs 1

I have a setup consisting of: 30' moniter, 6400+ x2, 4g ram, 1 8800 ultra and 32 bit xp and Im looking to upgrade to more screens. I have an identical 8800 ultra still in the box and 2 17' screens. I'd like to hook up the 2 other screens to my main comp, but with xp, the fps on one is always bad. So ive been thinking of different options:
1. Buy an identical 30' moniter, but will cost the most, vista
2. Hook up the other 8800, plug the 2 screens into the 2nd card and get vista
3. Just use one of the screens, one card and vista

As to my thread a few days ago, I just reformatted so I'm pretty much OK with upgrading to vista, but all I really want is to move more of my wow screens onto other moniters.

I appreciate your advice.

Last note, im a tech noob and when I tried plugging in the 2nd card with xp and then tried hooking up the 2 moniters into the 2nd card, they didnt register, what problem would this be?