wow, with those RAM prices wouldn't it be cheaper to just buy a second PC. you wouldn't even need that great of a video card. ( 8800 or 9800 would be fine )
I tried 5 wows on a single PC and I thought it was a little lag-ish compare to having two PC running 2 & 3 copies of WoW each.
I never have lag with two PC.

if your PC is using more than 80% (or 90%) of it's physical RAM you might start seeing page file swaps increase adding to lag moments.
I have 8G myself and never get close with only 3 copies of wow running.
I seem to remember that at one time I had all of the video setting set to maximum and each copy of wow was using 1.4G, it started to lag out and crash.