I doubt coverflow is supported on that card...
the old G3 B&W I had has a 128 rage (not the pro like you have though) and it doesnt do a whole lot of anything graphics wise.
Do you get the ripply effect when you add a widget to your dashboard?
Edit: just read the first posts, I could def be wrong... my post is almost worthless now, because I honestly am not a certain this is the problem as I was before when I read only this post...
Well, I can understand mine too, because my computer originally had a 450MHz G4 processor in it. The minimum requirements for Leopard are a a G4 with 867 MHz in it, which that model had a graphics card with 32MB of VRAM instead of my 16, so yea, that would do it.