just found a old ATI Rage 128 pro/ultra and tried to plug it in to the old mobo. The graphics card will initialize, along with POST: only one beep that means everything's OK. But no, there is no image on the screen...

This happened to me even in the morning with the "now-damaged" card: I saw nothing on the screen, and just after this black screen there was a colorful garbage so I turned off and on the PC and it worked. The PC isn't overclocked though.
Just curious, what may be the problem? Is the VGA badly inserted or such? I tried to reattach it 2 times and nothing... hmm should I use hammer? Nah. Should I reflash BIOS if I could find one?