My card, the Matrox Millenium G450 dualhead handles two monitors no problem (though there are few good dualhead games), the ATI radon series can handle dual display through it's TV or video output as well (but not in the same way).
I highly recommend a visit to
http://www.matrox.com as things have evolved considerabley since I purchased my G450, the new Parhelia series looks nice... To make the most of multiple monitors is a rather expensive proposition however - it is best to use identical monitors - but not necessary.
Mind you, some around here might appreciate these shots from some of the games enabled for the Parhelia multimonitor:
NWN:
IL-2 Sturmovik: Forgotten Battles :
Star Trek Voyager: Elite Force
Edit: but ah, I see you already have two video adapters... theoretically you should be able to install both, I'd use the AGP adapter as the main one and set each application to use the appropriate adapter as Towelie suggests. I've never tried that myself though. I'm not sure your system would be able to handle playing a game and decoding video at the same time... Perhaps a hardware DVD decoder card like the HollywoodMagic is in order, which I have used sucessfully in such applications (though now it has no drivers for XP...)