Your hardware setup is pretty similar to mine and I get acceptable performance running 5 copies of WoW.

Key things to do:

1. As mentioned, make sure each copy of WoW is running on it's own CPU core. Best way to do this is KeyClone, setting the CPU affinity in the "command" tab.

2. Are you using symlinks (Vista) / junctions (XP) for your Warcraft Data folder? This can make an enormous difference performance wise, and is described on the Wiki under "Operating System Specific Topics). In fact, I found a much bigger improvement in my performance doing this than moving to a solid-state drive.

3. You don't describe your monitor/account setup, but it's important for best performance to have your slave accounts use small window resolutions, decreased visual detail settings.

4. Have you set the maxfpsbk parameter in WoW to a low frame rate like <10? Again, the best way to do this is through KeyClone.