I guess you could dig up some old benchmark articles from, I don’t know, beyond3d.com or tomshardware or xbitlabs… Or just compare them by 3DMarkWhatever counts. Of course on a Mac they might behave a bit differently (because there Apple is making big parts of the drivers).
In general, both ship embedded (000, 100, 150), low-end (200, 300), mid-range (500, 600), and high-end (700, 800, 900) cards, with each plateau being worth a generation…
So a 6600 ~ 5300, an ATI X1900 would be roughly equivalent to an nVidia 8600 (X-4 = 7, so increase generation, reduce plateau).
This is all rough as hell, but it seems to hold up fairly well.