OK, let's try something different... and fail:

I have a PC X800Pro that I believe is in working condidition (I have no PC to make sure).
It looks like this http://microdream.co.uk/ati-radeon-x800-pro-256mb-gddr3-agp-video-graphics-card.html#.Vrouz_nhCHs
Try it in the G4 and it won't even boot (what a surprise, neither did the X1950GT:()
Try it in the G5 and it is not listed in pciscan output (but MOS did boot)

There must be somthing I fail to understand in AGP magic...
Should I try to reflash it?