peronally i would go no further than a 32. in my opinion your wasting money going any further. i bought a 48x for my pc and when i burn it says it gets no higher that about 30-40. I also have a fast machine and these are burns from the hard drive. I could be missing something vital other than that my lacie 24x seems just as fast as it. i think there is a site that will tell you what drives will work with mac os x. it is that accelerate your mac site, i went brain dead and forgot the domain name, somebody here will know it.
iJon