I have a laptop with a built in video card that is dual head. Which means there is only one video card with two display options for two monitors. I have windows 98 and windows 2k on it. The dual display on my laptop only works under windows 98.
At home I have two video cards. An ATI 7200 on my AGP slot and a Diamond Stealth on my PCI slot runing very smoothly on Windows XP.
My experience with dual monitors is that you have to have the right drivers for your operating system to recognize both monitors. On Microsoft's webpage they give all the supported video cards for dual display under Windows XP.
Check it out
http://support.microsoft.com/de fault.aspx?scid=kb;en-us;Q307397
Hope this helps.