You will need a high end add on video card. This also depends on how large a monitor you use as well. The Dell and Apple cinema 30" cinema displays require two outputs just to run the one monitor
You might try the nVidia GeForce 7900GT. This might be a bit much for your budget though. I'm not sure what cheaper cards support this feature.
Most mainboards (with onboard video) only allow you the option of using the onboard or a seperate card plugged into the AGP slot. Not both at the same time. Could be wrong about that.
You can find some decent dual videocards on eBay for between $50-$200. As Shaw mentioned nVidia is good.
I went to CompUSA and bought one of the CompUSA brand video cards. It has both a DVI and VGA port so I was able to hook two monitors to it. I got it when they had a sale going on and paid $30...I think the normal price is $50.
A good video card that offers dual monitor output if the Geforce FX 5200. It was an excellent card when it came out and it still works good. Obviously it's not top of the line anymore ( you know how technology is ) but I have it and it works great. I think it's between $30 and $50 dollars on Newegg
Are you saying that your mainboard has video on it or that there is a videocard already inserted into the AGP slot? Have you opened the case to see your AGP slot? (Note: If you have a new computer opening the case might break the warranty.)
EDIT: Found this image on the net. The AGP slot is #4. If it's available then you just need to buy a dualhead card and place it there.
I spent last weekend cleaning out all of my computers for dust, adding new RAM, cleaning the fans, adding better fans, adding DVD drives (which is much harder than it would seem)...
I don't even wanna peak inside one of my computers right now.