Well, basically, the way PCI slots are desinged, even on a faster 66MHz slot, you can only get a certain amount of bandwidth through the board. something on the order of about 500/sec. Which, really, is a lot. But it isn't enough for Quartz Extreme, the new graphics rendering engine for 10.2.
Quartz Extreme requires an AGP 2x slot at least. AGP has a bandwidth that far exceeds that of PCI graphics - about 1GB/sec for 2x. The standard is now 4x, which is you would imagine, much faster. The other problem lies in that the PCI graphics can't access the main RAM directly; AGP can. This makes it much faster for information to get to the AGP graphics card as it doesn't have to be passed to the processor to be redirected to the graphics card.
So there you go. PCI graphics are fine for 3D gaming from about 1998 or 9 at the latest. But AGP is where its at baby, and 8x is right around the corner. that's when i'm getting a new Mac.