i need to install a raedon 9000 pci video card but my dell dimension 2400 has
a on board intel how do i disable it so my computer only accepts the pci card

In the BIOS is the usual place.

You will need to press a certain key to enter the BIOS set up, this key
is usually Delete, some times F2 or F3, it will depend on what Del have
done.

You need to do this before the POST starts, whatch the screen carefully
when you start up you will usually be told what key to press, either
that or read the documentation that came with the machine.