There should be a BIOS setting to disable onboard graphics.
The only time I have done this was with an AGP card. I changed the graphics driver to standard VGA, shut down, installed the new one, then changed the BIOS as it booted for the first time. I then followed the manufacturer's installation instructions for the new card.
The newer generation of cards aren't my forté, so please...
Post your system specs so the hardware gurus know what they're dealing with.
e.g. Processor type and speed, Memory size, Motherboard make and model, AGP type supported
Make sure you don't buy a new card that needs a higher spec than you currently own unless you plan to upgrade again very soon.