There are indeed many versions of Garamond.
Apple's Garamond, the one you see on all the adds and on the boxes etc.., has been designed for Apple and is only for there use. You cannot buy it. Although you a quick search on Hotline will get you a copy. I have a copy, but then I used to work for Apple.
There are often replicas of fonts provided in the OS, for instance NewYork is Times.
A quick search of the web should turn up many free fonts, a lot of which will look like Garamond. Also if you have a copy of Fontographer you can convert your PC Garamond to a Mac postscript font. Although that might be illiegal....
spike.