> The 4850 did not give any output (granted, it may be dead).
> I also tried an HD5450, no output either.

It's kind of a gamble to have Radeon HD cards working on G5 Macs (PCIe and AGP alike), especially with MorphOS (Linux has some better support in this regard). Some cards work and some don't, even with the same chip on it. A pattern is yet to be discovered. So if you tried to no avail with holding 'C', then your cards are unsupported as of now (or dead). There's also no guarantee that a random X19x0 with AtomBIOS will work, but chances are much higher.