Hoyas!

Alright...here's the deal. Started two-boxing a couple of weeks ago on a single monitor, and had decent (30ish) FPS while twinking an alt. About a week after that, my wife and I received new 19" widescreens, and we proceeded to set both of our computers up with two monitors. Problem? Now my FPS is down to about 25-ish on a single client, and running two clients hovers anywhere from 5-15 FPS, and usually runs on the lower end of that spectrum. This occurs even with no programs running in the background.

Here's the system specs...

Dell Dimension DM051 (yes...I *know*...but it's what we could afford a year ago)
Pentium 4 CPU 3.00GHz
2.00 GB RAM
ATI Radeon X600 256MB video card
Windows XP

One monitor is a standard Dell 17" flatscreen, the other is a 19" widescreen (both flat panel). I only really need one monitor to be used for the purposes of playing WoW; I don't really mind tabbing back and forth between the clients, since this lets me use the second monitor for looking up quests, viewing the vent window, IM's, etc. The desktop is extended across both monitors, but it doesn't matter where the second client is located; FPS is shot all to hell and gone.

All other equipment is stock, I haven't made much in the way of upgrades other than the increase in RAM to 2GBs.

Can someone who is way more knowledgeable than I help me out?
If any more info is needed, please let me know.

Thanks!