My system is nearly identical to yours. I'm running 3 accounts, and I went the extra step of installing 4 separate SATA drives into my machine. Full copies of WoW on 3 drives, and the 4th for writting FRAPS when I choose to film.

I also set the video settings of my two "slave" copies to low on everything that wasn't nailed down. The slaves (running at 1280x512 off the 2nd monitor port of my HD 4870) actually load into zones/BGs/etc a lot faster than my main (running at 1920x1140 on the main port). The missing 60 pixles in my main is so that a double-height Windows Task + Quick Launch bar remains visible.