I'm running this on a G5 iMac 2 gig machine and it is excellent. Yes, you do need to muck around with ini files and it does help to read the forums.
Some cars are great and others hopeless and the same goes for tracks. It seems better to go for new tracks and cars as they are optimised for newer versions.
It also networks nicely with a PC I have, but again you need to muck around with the ini files.
It is quite realistic and Rudd has done a great job.