: I didn't touch the Open GL settings, I have a 16 MB Radeon 128 (700 Mhz iMac
: G3, 256 MB ram, OS X 10.4)
Ooh, that's almost as bad as my old beige G3, which didn't even have an OpenGL-compliant graphics card (8MB seemed huge at the time).
The bottom line is that you probably don't have enough combined RAM and VRAM to load the high-res textures. When AO loads up, it loads as many textures into VRAM as space will permit. The rest it loads into system RAM, swapping textures in and out on the fly as needed. If you don't have enough room in system RAM either, I'm not sure what happens - theoretically it should swap out to your hard drive, but that would drop your frame rate to an unacceptable low. Perhaps it truncates the number of bits used to render the textures, leaving everything dark, but that's just a guess.
Bottom line, my friend, is you probably need to stay away from the high-res textures until you have a better computer.