I've just upgraded my multiboxing rig and just thought I'd feed back some info on how well it performs. Might help other people decide on upgrades.

I five-box on a single PC across two 1920x1200 displays. My previous PC spec was: Core 2 Quad Q6700 (2.66GHz), 8GB DDR2-800, and a GeForce GTX285.

On that machine, with my non-active windows capped at 15fps and all graphics settings at minimum, I got a playable frame rate on my active window out in the less busy zones, about 40fps but rarely 60fps. Dalaran was pretty unplayable, with only about 10fps in any windows. Running around the city was quite laggy and the slaves would occasionally break follow.

I've just upgraded to: Core i7 920 (2.66GHz), 6GB DDR3-1333 (@1066MHz), and kept the GTX285.

I've been able to increase the background FPS cap to 20 and increase the view distance (farclip) from 177 to 800. My active window now stays at 60fps nearly all of the time and Dalaran is a much nicer experience. At peak usage, about 5.5GB of my 6GB of RAM is used, and that's with the texture cache on each WoW instance set to its maximum of 512MB (default is 32MB). Here's a screenshot of machine load while in Dalaran.

Obviously the new CPU and memory architecture has probably made the most difference, but also putting my GTX285 in a new motherboard that supports PCI Express 2.0 has probably helped too.

I also highly recommend the TweakWoW addon. It gives much finer grained control over game settings than you have in the various Video/Sound/Interface options menus, and most importantly it lets you enable a silver or gold dragon around your unit frame