> I've got a HD4350 PCI, which I tested in my Pegasos II.
> OF obviously doesn't work, but MorphOS 3.10 should.

Should or does/did? Initial HD4350 support was added with MorphOS 3.8.

> Would be fun if someone could try an AGP to 3.3V PCI passive converter [...],
> and try the PCI Radeon HD in the AGP slot

Yes, as the weakest link determines the total speed, PCIe <-> 66 MHz PCI <-> AGP should be twice as fast as PCIe <-> 66 MHz PCI <-> 33 MHz PCI.