perhaps a silly question - but can you boot into any OS ? a winXP CD perhaps ?

I ask because, i wonder if you have disabled the Onboard Graphics in the BIOS - and had you successfully removed the Drivers ? (in windoze Add/Remove Programs in Control Panel) -- you said that it boots fine previously, before the GeForce 5500 Adapter install...but don't mention whether or not you can successfully switch back and forth via Enabling/Disabling Onboard graphics in the BIOS.

You should be able to boot into a standard VGA display adapter without Either the Onboard graphics or the Video Card installed (BTW - the 5500 is AGP, yes ?)

Do you know how to uninstall old and install new hardware ? ...In windoze, you would boot into safe mode and remove the Onboard graphics accelerator drivers from Add/Remove Programs, as i noted above -- then shutdown normally, Unplug from wall (or pull ATX power cord out of back) , open the case and install the Video Card (Display Adapter/GeForce FX5500).

Then plug in, and Boot directly into the BIOS and;
* Disable the Onboard Graphics system
* Set "Primary Display Adapter" as AGP (for an AGP card , PCI for PCI card)
* check the aperture size and other AGP port settings, like voltages
* Exit the BIOS Saving Changes....

Upon Boot, you will/should be prompted for the Drivers for the New Video Card -- or you can elect to just use the Standard VGA adapter