Look in your BIOS to see if there's an option to disable the on-board graphics. If not, check to see if the first VGA device (may be slightly different name) is set to AGP (assuming you have an AGP card). After that, put in the new card and see what happens. You should have video on the new card.
You also may not have to do anything, as the on-board graphics may disable itself when using a different video card.
If you run into trouble, drop by again and give us the low-down on your specs:D
Also, please do not double post in the future. It's hard keep track of which thread was replied to. Don't look for the other one, as I already deleted it. Thanks:)