lol well idk, but I can tell you I run 4-5 on 1/2 that.

My rig is
2gig
6800gt 512mb
2.4 single core

I run my 4 shaman (completely min video settings and a minimal UI, 600x800) I run these 4 around no problems untill I run into a large crowd (I.E. Org/shatt)

I also run instances with those 4 off on my 19" LCD, and a 70 holy pally, normal maxed video settings and my normal raiding UI, on my 21" LCD and I run them through instances no problem, I get maybe 20ish FPS (not awesome, but not bad)

I also don't use maximizer, which I think I will soon b/c it will help me with FPS.