OK, this is coming completely from experience with computers, and hardly any experience multiboxing

NVidia and ATI software HATES onboard graphics... If you randomly have a 2nd monitor and only a single head graphic card, its a mess but if you can installl the driver without the other software, you can dual screen with that set up. If you are using this setup, upgrade before you start getting serious about trying stuff like this.

DirectX didn't add acceleration to a 2nd display until either DX8 or 9. So you're fine so far, but XP still limits resources to a 2nd display, so running WoW on a 2nd display in XP probably means its not working. Some people can get away with running WoW in OpenGL on the 2nd display and it works perfect (Right click the launcher shortcut -> Properties... at the end of "Shortcut" add -opengl (ex. Shortcut: "C:\Program Files\World of Warcraft\Launcher.exe" -opengl)) Downside if this works... you cant use the hardware cursor anymore (probly not using it too much anyway) and WoW doesn't look as "pretty", but performance > graphic quality when you're doing something like this.

Your best thing to do, no matter what, is upgrade your hardware. But before you do that, you gotta really look at what youre trying to do. You can buy a really good card for $250 or spend $100 more and get a new cheap computer... The way computers are now, a $350 computer will run WoW surprisingly good, i would recommend upgrading the RAM and Video card with the computer (about $500 total (GeForce 5series works more than good enough for WoW)). But the 1 thing you have to look at before you do this, is your internet connection. 2 computers take up more of your bandwidth than 1, you probably wont notice too much of a difference if you have DSL and ONLY 2 computers running in your house. But a 3rd and you'll start seeing some horrible lag...

Or the easy fix for now, turn on horizontal span and learn how to use maximizer, true, your desktop will look pretty bad if it looks too cool on 1 monitor, but you care about getting WoW working