I thoroughly agree with the others about the quality of this game. I have an iMac 2.8 Ghz with 2gb RAM. I don't appear to have the graphics issues that others here have mentioned however I do have one small gripe with this game. Gamepad support. If you prefer the gamepad over the keyboard/mouse approach then you may be in for a (albeit small) disappointment.
There is an option to use a gamepad but it is for an Xbox 360 controller only and doesn't work with any other device. I also own a PS3. The PS3 gamepad doesn't work of course, neither does my LogicTech Dual Shock gamepad that I bought especially for my iMac. The PS3 gamepad is now supposed to work (via USB) following a recent update to MacOX, but I still can't get it to work properly. Oddly enough I've seen reports elsewhere that even the Xbox 360 controller doesn't always work too well either. By way of comparison, Tomb Raider Anniverary (also Feral) works well with the LogiTech Dual Shock.
That said, they (Feral I think) have promised an update for the Logitech Dual Shock immininently. But that was months ago and the auto-update feature still finds no update.
Overall a very good game but if, like me, you have carpal tunnel syndrome or simply prefer using a gamepad, I would check with the publisher first if I were you. I suspect Bioshock 2 probably will fare better in this respect.