well try this
1.make sure its seated properly in the AGP slot.
2.try running hardware detection?
Go into Device Manager, uninstall the device, restart PC, use Add/Hardware Wizard from Control Panel use the onscreen wizrd to select the type of device.
if that doent work try this
3.try booting with the old video card, then install just a Standard VGA driver in place. Then shut down, take the old card out, put the new one in.
4.If that doesn't work go in BIOS and say "Force reset ESCD database"
5.you may need to manually configure the settings for the video card. IRQ, DMA etc.
6.you should check in the BIOS to make sure that you have teh AGP settings correct. If it supports 8X make sure thats selected, and the AGP aperature size should be around half your RAM or 128
these are a few things you could try and ill do some more resaerch and hopefully we can figure this out let me know how it goes