I see.. I have a couple more ideas for you if you'll try them
Couple questions first,
What CPU do you have?
What speed do you have your memory set to?
Are you overclocking? (You might be and not know it :P)
If you could let me know the specs on your CPU (speed, fsb, which core it has, etc.) and tell me what speed your ram and bus are at, that would help.
Something you can try meanwhile is set your bios to "Optimal settings" or "best performance" and see if it boots up. It is possible that one of your bios settings could be causing the card not to work.
For example, it's possible that if you're using some non-standard bus speed the AGP bus would be running at a speed that is out of spec. Another thing I wondered is, you have PC3200 RAM which is supposed to run at 200 mhz * 2, but lots of Athlons don't really support running the BUS at that speed, and most Athlon boards I have seen run the bus and the ram at the same speed so if you run the ram speed 200 mhz the bus is also running 200 mhz, which is overclocking the CPU quite a bit if it's really a 166 or 133 mhz bus CPU (I think they call it 333 mhz and 266 mhz though when the CPU specs talk about FSB because they are counting the speed * the 2 channels).
Anyways let us know... Maybe you can still return the new motherboard if we figure this out...