perhaps you have a mobo that can't accept an agp 4x/8x card.
I just recently bought a new video card, an ATI Radeon 9250 256mb AGP card. My old card was an ATI Radeon 7200 64mb AGP card. When I went to switch them out my computer didn't recognize the new one yet it still runs fine with the old one. Basically my monitor never recieves a signal and it gives me the error sound of a long beep followed by 2 short beeps. I hooked the card up to another older computer running a Nvidia GeForce PCI card and it booted up fine. The old computer is 5 years older than mine. I've tried everything I can think of and I'm stumped. Any help would be appreciated.