Was the iMac powered up while you were testing the Beige's video with the display? If not, try again with the iMac powered up. Or, try hooking up the LCD to the iMac and booting that machine. I don't think the stock video on a Beige G3 will handle 1280x1024. The Formac card, yes, it should've worked. It would be slow without the drivers installed, but it should show video. Otherwise, try a Radeon 7000 or 7500, making sure that the operating system is recent enough for the card.

-J