Hi iztehzha,
I'm running a very similar box to yours.
My setup is:

AMD Phenom X4 9850 (2.5GHz)
ASUS M3N78-EMH
Corsair XMS2 DHX 4GB DDR2 2X2GB DDR2-800
EVGA 8800GT 600MHZ 512MB 1.8GHZ

I only have 1 500GB 7200RPM Hard Drive in my computer right now, but I am able to 5 box just fine. I am getting about 30-45fps on my main @ 800x600 will full detail.
My setup has problems in town (Stormwind is the only major city I've been to thus far) and this would probably be improved with more RAM (or 64bit Vista) or 2 Hard Drives.

Your setup should be fine to 4 box.