I had the same problem in XP until I switched to SPAN and it was like night and day. Went from like 5-7 FPS to 30+ sustainable over 4 additional WoWs. That system was running a Q6600 and 2 x 8800 GTXs.

I'm using Windows 7 now (starting new team, on new PC) and I'm pleased and not noticing the same issue. Long story short, shadows and SPAN vs. Dualview are your most likely culprits, seriously. It was really annoying switching modes every time, but it was absolutely, 100% worth the performance increase. It was not small.