Yep, my system is almost the same as yours except I upgraded the E8400 to a 9550, and put 8 GB in it with Vista 64(because it has much better dual monitor support).

I run 5 clients, and the 2 sharing a core usually show lag before the others. The 8800GTS is fine with my 2 monitors, so long as you drop the anti aliasing level on all clients (and you'll have serious FPS issues if it doesn't match between clients), along with a few other settings. Mine is running all 5 clients at 1920x1200, and the main window is usually between 30 and 80 FPS, while the other 4 are limited to 30.

I've yet to take all 5 to Dalaran (waiting on TBC and WotLK for the last 2 accounts), but Orgrimmar at peak times isn't noticably slow with 5 at a time. The biggest delay I get starting up is the 5 copies of Questhelper (when all I need from it is the tracking of who has how many for collection quests). Symlinking makes loading a LOT quicker.