64>256 is my way of saying to change the 64 card to the 256 card you need to change from pci to agp?
Sorry to confuse you.
Anyway, it shouldn't really matter, the bios should read whatever card is plugged in, but you never know.
I'm thinking as a test you should get into bios with the 64 card, change the bios to agp, save then turn off the computer, then put in the 256 card & boot to see what happens.
Only that you say it works in a different computer I'd be thinking the card is zapped.
Have you tried it in the other computer again to be sure?