I just got my system up and running. I had some problems with my dual-vid card setup under XP so I switched it to a single card using horizontal span and it seems to be working well.

My setup is:
Quad-core Q6660
4GB ram
8600 GT (I also bought a second card, I didn't end up using, and I'll return)

I bought the full system in the 950$ range, including shipping, not including monitors, which I already had.

I got it running yesterday, using the single vid-card, using horiz-span, with my main on a single monitor, and the others quad-screened on a second monitor. By tweaking the graphics settings, I was able to get them all running in the 40-60 fps range.

My experience matches yours, very specifically, tweaking video settings has a large impact on fps. My CPU usage hovers near 50-60%. It's very obvious that my framerate is determined by the vid-settings and is GPU bound and it seems to me you could get away with a near-top-of-the-line dual-core setup. If you really want high framerate, you need a good vid card. I'm getting 40-50fps with a 120$ card. My graphic settings are pretty much default on main (and roughly full-screen reso.. i think it was 1024x), and minimum on others (800x600, everything off/mininum except terrain distance which I have set in the middle).