Onboard graphics card replacement problem

HI! Your assistance and suggestions please! I want to replace my old onboard graphics card so I purchased a PCI-e Graphics card which is an ATI Xpress Graphics with a new NVideo700 Series card. The problem is that when I turned it on, the boot up was unsuccessful. Doing it in BIOS also doesn’t work. I tried to unplug the card and went back to the original onboard graphics and it worked very well. I don’t know a way and I didn’t dare to try to disable the onboard graphics in BIOS without using the jumper links, assuming that it will run or operate automatically. Kindly give an easy-to-follow procedure to make it work. Thanks.
