All modern Macs are PCI. This includes all the iMacs, though from what I understand the original iBook was not.
I have run Rune on my iMac DV 400 with 192mb of RAM and I can tell you that Ca$h's estimation is more or less correct. It runs fluidly, though not spectaularly.
If you want the "Wowzers! That looks photo-realistic!" graphics, you'll be disappointed. Of course, if you want a "serious" gaming machine you can either buy a G4 tower with that NVidia card or slap together a PC.
Question-on which graphics engine is Rune based?