I currently use a Radeon 7500 in my souped-up G4 Cube which can display 1080p just fine under os 9 (9.2.2) and OS X. My screen is hooked up to the VGA port because I can't seem to get the digital signal to work properly with the ADC->DVI adapter (not sure if it's the adapter or my monitor). The original Rage Pro 128 which came with the Cube was also able to display 1080p albeit only on the VGA port (analogue) and somewhat "dark" color-wise (I suppose the resolution was pushing the hardware). The digital output on the Rage Pro 128 is limited and will only give you resolutions up to 1280x1024 or thereabout. The Radeon 7500 should technically allow you to output 1080p on ADC/DVI.

Trying to get a digital-only configuration working on my Cube so far has been an exercise in futility. If I hook up my screen to the Cube using the ADC->DVI I get no video output at all. If I connect two screens to the Radeon, I can't get a signal on the digital connection but the VGA port outputs as secondary screen, so I have no ability to actually control the system (all I get is a blue screen showing the desktop pattern). If I boot into OS X using this configuration, once the OS loads it will actually enable video mirroring which will allow me to modify the screen setup (once I do so it displays 1080p on digital just fine). Again, this only works with with OS X, can't seem to get it to work under OS 9 at all. So at this point I'm stuck with either an unusable dual screen setup or 1080p on analogue since digital won't play nice.