Screen stuttering on dual monitor set-up

Okay, so my rig is listed below:
Q9450 Oc'd to 3.2
4 gb of ram
4870 graphics card
asus rampage mobo
windows xp 32bit
some WD HD's (2 500gb)
Dell ultrasharp 24" monitor
LG 19" monitor

I've noticed that when I play anything game wise on the 24" (mostly warhammer online recently) and I play it in windowed mode, when I move to an application on the 19" monitor like vent or an open firefox browser i get ALOT of stuttering on the main monitor.

My question is if this is most likely because the 4870 can't handle the total screen real estate or if i have another problem somewhere.

I'm thinking it may be the card because in large keep sieges on warhammer i get some serious frame rate drop but we are talking about 50-60 people on screen at one time on a really large monitor with all the graphics turned to max and all sorts of special effects going off.

Maybe it's time to buy a second card and x-fire them.