You have to bee very careful when doing this. As stated in the previous post, some cards just don't work well together.
The best way to do this is to get a card with multiple outputs. Matrox has been the standard for this for quite some time, but they aren't made for gaming.
If you want to use an old card, make sure it is PCI and your primary one is AGP. Many people have gone through a lot of problems with driver conflicts. Try checking out the following site to see if your two cards work well together.
http://www.realtimesoft.com/multimon/db.asp
Here is an article by The Screensavers that you might want to check out.
http://www.techtv.com/screensavers/a...342609,00.html
Good luck.