I currently run 1 or 2 gtx 470s in sli or surround with 1, 2 , or 3 monitors depending on my mood.
Normally I run games like PS2 or Blops2 on the main screen and the second screen has steam friends, core temp, afterburner, and maybe even an avi playing in VLC. I notice no frame dip at all. If there is one it is around 1%.
All 3 screens are at 1920x1080 at 60hz.
Running games in surround, now that is a different animal. running 5760x1080 cuts the FPS down to about 30% of what it is at 1920x1080.