The way a Pocket PC uses memory is very different from how a Zire does; the 32MB RAM in the Axim won't get you very far in terms of added applications. That's why an additional memory card is (IMHO) a necessity.
The most efficient way to use the card is to install most of your applications onto it, like the hard drive on your PC. That leaves as much of the internal memory free for actually running the applications.
Finally, there is a program for synching with a Mac. It's called Missing Synch and you can get it here:
http://www.markspace.com/missingsync_pocketpc.html