I have on-board video that I use now but I have an AGP video card that's just sitting on my desk as well as another monitor. Could I install the card and have two working screens side by side? The on-board graphics is nVidia I think and the card is ati radeon 7500 if that matters at all.

Nah, this won't work. Usually, when you plug in a dedicated AGP card, the integrated card disables itself. On old mobos, you had to do the choice manually with jumpers but it was always the onboard card OR the AGP card. Never both. Sorry.

If you want dual screen, you'll have to get a graphic card with two screen outputs. It's getting more and more common nowadays.

Okay thanks. I got a new video card for my other computer, its PCI-EX, in order to do this. Now my question is why do things like games and video minimize when I do something on the other screen? I want to have a game open and running or a video playing and still be able to chat, browse or whatever on the other monitor. Any suggestions as to what to do? I've got them setup as two separate displays.

Alt+ Tab is your friend.

Doesn't work. Game minimizes when fullscreened or freezes while in window mode and video freezes with sound continuing.

You can depending on BIOS, drivers, and operating system.