Sli or CF won't help you. but it might be easier for us to help you if you tell us what your FPS are with no lag and what it drops to with the lag.

like many other posters have said before. drop your shadow to lowest setting and lower the slave settings to min. but my guess is the real bottle neck is in your ram - HDD exchange. if your in large population areas and your FPS is over 20 than i would say that is avg. Your video card should be able to handle any thing 5xwow can throw at it. there are plenty of multiboxers here that are using an 8800gs and don't have lag problems.

also consider your wow server, some of the lag may not be in your control but a factor of your internet connection, distance to wow server, wow server lag and data threw- put. 5 instances of wow require 5x the data to run. so any delays in this from your

hard drive -> ram -> cpu -> ethernet card -> ISP -> wow server -> ISP -> ethernet card -> ram ->cpu -> gpu.

the gpu is the last in a long chain of events to play the game.