Have you tried with a display connected to the onboard graphics rather than the graphics card to see if it outputting the display to there instead? The onboard graphics on these machines causes nothing but problems, OS X (especially Leopard) hates it, and I have seen these machines decide to use the onboard graphics even when there is only one display connected and that is to the graphics card!
If you haven't tried that, you could try holding the combination to get into Open Firmware for long enough for it to have done it (I have had to do this before, where I can't actually see the output as it is going to the useless onboard graphics), type:
setenv pci-probe-list fffbffff
Press Enter afterwards, then press Ctrl + Command + Power to reboot - the command disables the onboard graphics for the next reboot, so forcing the output to go to the graphics card. There is a more permament version of the command which persists until the PRAM is reset if you need it (mine kept freezing after a while in Leopard without doing this, as the onboard is so incompatible with OS X).
Though I only remember seeing this with OS X so might not help you, but worth a try - I have found XPostFacto to be much more reliable for booting OS X (even supported versions) on these machines as it allows you to set the graphics card as the output, whereas without it the machine always seems to want to use the useless onboard graphics.
By the way your machine will run Leopard if you want it to as you have a G4 processor installed, it's not especially easy to install but it does work nicely once you get it on there.
