I have Mac Pro 8 core, 8 gigs ram, nvidia 8800.
I will say though, I notice way better performance on the PC. I am using bootcamp with WIn7 64bit

I also have an Alienware Quad (pre I7) with 4 gigs and nvidia 8800 that seemed to out perform the Mac Pro, but I recently added a second video card to my mac (lower grade nvidia) so I could have 3 monitors running. Makes working easier.
And gave my daughter the Alienware.

I have run 6 accounts on the Mac Pro, all on one monitor though (30 inch), not that breaking the screens up between 2 monitors would matter much. The 6 ran pretty smoothly. I want to try 10 boxing though I don't think I am that good with 5 so I cant imagine 10.