Pally and 4x Shaman would be a good start. I run 5x Shaman and 5x DK atm with plans on a diverse group in a couple months (Pally, 2x Shaman, Warlock, Hunter) possibly.

I am currently running 5 accounts on one computer (2x 2.8ghz cpu, 2gb ram, GeForce 7600 GS) with minimal issues. Four accounts run with no sound and reduced effects while the main is sound enabled and higher settings.