I run 5 on my pc as well. I used the technique in this link to symbolically link 5 directories together. Essentially it allows multiple directories to run the common files from 1 directory. It lightens the load on your HDD and your PC in general. I really think it is the key to getting good performance from a single PC running 5 copies.

PowerShell script to create symbolically linked (mklink) copies of WoW

It also allows for only needing to update one set of mods/addons while keeping the custom settings seperate for each directory. It's a little techy, but its worth trying.

oooh, forgot my specs

c2d E8400 processor (3GHz)
4gig ram
1 380g HDD (7200 rpm, nothing special)
nvidia 8800gtx vid card

I can run 5 instances of wow across 2 monitors with Winamp and a Firefox open in the background. I have almost no problems except for high pop cities like Org.
I have my FPS capped at 55 on my main window and at 20 on my slave windows on the second monitor.