ok im running 2 instances of wow with up to 60FPS on both running them dual screen with full graphics settings and full AA and AP filtering on

never had a problem computer handles it fine,

pc specs are

Core 2 Duo E6600 (2.4 ghz)

2 gig ram

XP home edition

Geforce 8800GTS 640mb

i think your graphics card is the main factor here you need something powerful to handle 2 3D applications at once

i regularly do boring grinding on wow on my secondary monitor and wathc a film on my main monitor without any hiccups or lag.