I'm building my wife a new comp right now for the same purpose. So far here are the ingredients:

AMD Phenom 9500
8600GT 512MB (Main monitor)
6800 512MB (Secondary monitor, I own 5 of these already, so they're getting used)
4GB RAM
Dedicated Raptor
2x 22" Samsungs

I figure it shouldn't have any issues running 5 accounts. I run 2 on my FX60 and only use 50% of system resources.