I currently have dual monitor setup, running Vista 64 on a Core2 Quad with 4GB RAM. My current video card is a Nvidia 8800 GTS 320MB driving two Acer H213H @ 1920x1080.
I play WoW, and when I run it in full screen mode, I average 50+ fps which is fine, but if I run it in windowed mode (where I can move my mouse to the other screen to interact with a browser for example) the frame rate drops to around 20, sometimes into the single digits.
In either mode, my CPU utilization is around 25-33%, so I do not think that it is the bottleneck.
I'm thinking about upgrading to a ATI 4870 card with 1GB RAM, but I'm open to suggestions for cards under $200. I do not have a SLI MB.
I found another interesting tidbit of information as I was searching for an answer to this. One forum post found mentioned turning off Aero interface option in Vista.
That has made a significant impact already. I lost the pretty transparency effects of the UI, but gained about 15 fps in windowed mode, my CPU utilization also seemed to drop, so both the CPU and GPU are hit by Aero UI.
I'm still thinking I want a newer video card, it has been 2+ years since I got the 8800 GTS.