I have a similar setup. Try browsing my posts to see what has / hasn't worked for me, especially:
Need some help 2 dual-screen mode + vista 64 problems...
Adding 3rd video card = *worse* performance?
[Other] PowerShell script to create symbolically linked (mklink) copies of WoW
Reducing latency

I also suggest using perfmon.exe to see where the bottleneck is.