I have 3 monitors, but 2 of them are on a crappy HD2400 pro so are used for browsing, TS etc.

all 5 instances are on 1x 24" running off a Radeon X1950XT. My setup is 1 big wow @ 1538x1200 and 4 smaller ones stacked vertically on the RH side @ 384x300. This gives them all the same aspect ratio and PiP switching is very quick 1/5th second max.

I run my main WoW from C: and the other four from D:
I have optimised performance on D: by having one full wow installation (copied from the one on C:) lets call it WoW2
I then copied everything from WoW2 to WoW3,WoW4 and WoW5 EXCEPT the Data and Patches folders
I then linked the data folder from wow2 to the other installations using junction (on vista use mlink - search the forums for instructions)
I think I could get better performance again if all of them were on the same disk. (more caching)

I do get some slow downs in cities but only while loading textures (Al'Akir is a busy server) once it has stuttered a few times though it improves again. out in the wilds and in instances it flies with 30+FPS on my main. The small clone windows help kept overhead to a minimu