When your computer is starting up, keep tapping the F8 key...
You will now be in the BIOS...
Naiviagte using the arrow keys and to change the settings use the page up and page down keys.
Look for something like "ONboard grpahics". Use the arown keys to go to this option and press ENTER. Use the page up or page down to disable.
Go to the EXIT and make sure it says SAVE changes then press enter.
Pop the new card in.
If there is no option within the BIOS to disable the onboard graphics. Post back.