Hey Paul, since you have a lot of extra speed, try the following settings:
"High" texture quality
Everything else in the game menus maxed or enabled (except VSYNC and FSAA)
In the ATI Displays control panel:
(make sure that you don't force Quake 4 to disable the AF)
Commentary: first of all, the shadows are a definite must. The main reason why Doom 3 and Quake 4 are considered groundbreaking in terms of graphical quality is because they feature dynamic lighting, which no other game on the Mac had until now. Second, the Doom 3/Quake 4 engine is a bit strange in that there's a significant increase in visual quality that occurs when you reach past 1024x768 resolution. At 1024x768 and below, everything tends to look really grainy. The reason why I specifically suggested not turning off AF is because you really, really want that enabled. Without it ("High" defaults to 8X), everything you see will look blurry until you're right on top of it. The higher it is, the farther away you see the maximum quality textures.
Lest you worry about performance, my system does great on these settings (minimum framerate is about 25 fps, averaging about 38-40 fps).