I just recently started dualboxing, and what made it real easy was already having a beefy system. I bought a Mac Pro earlier this year, and with 8 processor cores, the CPU side is never a concern, even when using the built in video recording function of WoW. Graphics wise, the box also has 4 16x sized PCIe slots allowing for 4 total cards (though only 2 6pin power connectors). For now, I ensure both clients get full GPU power by running an 8800GT attached to one LCD, and the 2600XT that came with the system attached to another monitor. OS X allows the WoW game window to run on either card, and I can even drag the window back and forth between the two screens without any issues. The GPU that has the most pixels of the game on it is doing the rendering for it. The only downside is rebooting into Vista to play some Windows only games blanks out the second monitor, as it can't support two video cards that run different drivers. XP can, and apparently Windows 7 will be able to again, but for now it means it's not real easy for me to start dual boxing a Windows only MMO.

And welcome back from Blizzcon. Was running around there myself as well, and had a blast playing Starcraft 2.