Your max polys per mesh is only at 9.7mil probably why your getting the max at about 8.

Theres a thread at Zbrush central that goes over getting the best out of the PC.

From the memory scan it looks like your full up for the PC. Not too sure but in XP you used to have the "3gig switch" to imporve programme RAM usage, maybe its in vista too? I'm on 64 Vista on all my comps, which when you swith off all the crap is great.