I use two computers. I have a year old single core processor computer, and a year old dual-core processor computer. Both purchased on Craigslist.org for cheap, updated their video cards, and put 4 gigs of RAM into the dual-core. All together, with two 19 inch monitors, one wide-screen and one just an older HP LCD 19" Flatscreen, I probably spent about a total of $700 US.

I get 60 FPS on my single core, which I have my Main on, and about 40 FPS on each WoW window on my dual core, playing four instances of WoW. When I want to do BG's, I use a main on my single processor, and three slaves on my dual-core. Remember, you don't need 40 FPS to play WoW. I've played with full settings and got about 20 FPS and never lost a character while doing long runs, and instances, terrain, and BG's all look perfectly fine for me. I use Synergy, and Keyclone.

I've never had any problems.

There are a TON of questions that are similar to yours on this forum, as it's probably the most common question too. You will run into lots of posts with people saying "Will this work" - listing about $1500 worth of computer parts, and they will work, but not a whole lot of people actually multibox on such updated systems.