Found this on Dell's sight. Should apply to your system.
http://support.dell.com/support/topics/global.aspx/support/kb/en/document?dn=1033493&c=us&l=en&s=dhs&cs=19How do I disable or change the onboard video settings for my Dell™ Dimension™ L series computer?
Discussion
The Dell Dimension L series systems have integrated video chips in the system motherboard. If you wish to install an additional video card in the system as the primary video card, you must change the System Setup video configuration. There is no option to disable the onboard video, you can only change which video chipset is the primary one.
NOTE: Before you proceed any further, determine whether or not the video chipset is integrated on the motherboard. Refer to:
http://support.dell.com/us/en/docs/index.asp. Click Dell Documents, click Systems, click Dimensions, and then select your system model from the list. The Technical Specifications will contain information about the video options for the system model. Refer to the section Removing and Replacing Parts for help with the physical installation of the additional video card.
Solution
To change your primary video from the onboard video to the PCI video card, perform the following steps:
Turn on (or restart) your system.
When the blue Dell logo appears, press the
key.
The blue Dell logo will change size and position and the message Entering Setup appears.
Press the key again if you do not see the message or logo change.
The Dell Dimension System Setup screen appears.
Press the key.
The Advanced screen appears.
Press the key to highlight Video Configuration and then press the key.
The Video Configuration screen appears.
The Primary Video Adapter should be highlighted; press the key.
Press the key highlight PCI and press the key.
Press the key.
The Save Configuration Changes screen appears.
Press the key.
The screen will go black and the system will restart.
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As RTR said, go into device manager and uninstall you onboard drivers before doing this. I'd uninstall the new card drivers also. Then reboot, then change the primary video adapter to PCI and install the new card drivers.
You may need to hook up your monitor to the onboard adapter to disable it then change it to the new one.