I use VNC to do hardware 5-boxing on 2 monitors. The four minion boxes have split-screen VNC sessions running over gigabit ethernet. The framerates are pretty lousy, but enough to be playable; when necessary, I can switch over to one of the minions and control it just fine. The important thing to realize is that even though the visuals look choppy in heavy action, you are still getting the full framerate of the machine behind it, so there is no delay in how your character responds.

I initially set it up as a temporary solution to see whether I liked multiboxing, before buying 3-4 more monitors, monitor stands, a new desk, etc. It's not ideal but it works well enough that I haven't felt any need to move away from it.

I'm (very slowly) working on a VNC-like program optimized specifically for multiboxing. I should be able to get significantly higher framerates than the regular VNC clients. I just spend too much time running instances to work on it

Edit: in terms of actual numbers, I'd estimate I get roughly 10 FPS on each of the four VNC sessions. However, the machines behind them are running at ~40FPS, so there is a huge difference between that and something that's actually running at 10FPS. I tried several VNC packages, and using TightVNC I was actually able to get almost 20FPS on each of the sessions. Unfortunately TightVNC's viewer app takes up a lot of CPU, which was bringing my main WoW instance down to 20FPS as well, which is not acceptable for me :P