I am running atm 5 clients on my machine (2.4 intel duo core, geforce 8800GTX 768 mb, 2 gb ram (will get more soonish ) with 2x 20" monitors. Binding them all with autohotkey + some other hardware. For now (5x 19 lvl characters) it works fine, probably will have to change something for lvl 70 pvp.

1 main window on 1 screen and 4 others on another screen. Getting some lag in capital cities, but outdoors and in instances works just perfect.