Hi, you're kind of in the same issue as me. I'm running 4 gigs of RAM, a 3.0 ghz Athlon and a 9800 gx2. Similiar setups.

I figured out that if I doubled my RAM and got a 64 bit OS I could probably do 3-5. The problem is the graphics are going to have to be on low and I'd be running 30 fps on your main and 10 on your slaves if I'm lucky. You also need to remember that when Cataclysm comes out in 2 months, multiboxing in Cataclysm with that set up will most likely not fly.

I came to the conclusion that it's not worth 200-300 dollars to upgrade a system to 5 box now when I could spend 1000-1200 and 5 box in Cataclysm.

Just my 2 cents.