Is the power button lighting up?
Might be time to replace the PRAM Battery, 4 years is close to their lifespan, far less if ever without AC power, & can cause strange startup problems...
Have you done a PRAM reset, CMD+Option+p+r...
How to reset the SMU/PMU on a Power Mac Earlier G3, G4, G5 models...
Does it boot into any of these modes?
Holding Option/alt key at bootup.
Holding SHIFT key at bootup.
Does it boot to Single User Mode, CMD+s keys at bootup, if so try...
Repeat until it shows no errors fixed.
(Space between fsck AND -fy important).
Resolve startup issues and perform disk maintenance with Disk Utility and fsck...
We might get clues with verbose mode...
Be sure the Personality card in the PERCH slot is fully seated. They can loosen when doing other work insde the computer. Some of the Beige old timers always recommended removing the P-card, cleaning the contacts and blowing out the slot rather than simply check that all the connectors are parallel in the slot,
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.
Allan, for some odd reason I don't think I'd considered that... although perhaps it was because at the time I didn't get near the card, I don't know. I might give that a try sometime.
You're right TDB, and I actually DID try that. It worked, but the thing still refused to boot even to a CD-ROM (even when I reinstalled the original 24x Apple/Matshita). I just hope that means what we think it means, and not that I fried the original Radeon, which are now pretty rare and probably collectible given they're the original Radeon PCI w/fully working fans and OCable to ~200 MHz core/mem, a 20% increase
I'm pretty sure I just fried it, which is really too bad, I loved that old computer, and it had been going strong for fourteen years; I'd even made custom mods to theh chassis haha
I dunno, some day I might try on it again. At the moment I'm playing around with an Early 2011 MacBook Pro I bought for my girlfriend, but as of tomorrow that'll be in her possession and I won't be able to use it anymore
*Sighs* Back to Windows 7 for me :/