Optimizing for audio?!? This IS a Mac, after all ... ;)
Seriously though, there's not much you need to do to optimize the mac ... you *could* do little things like ensure background processes suck as dashboard are not running and consuming system resources (use the Activity Monitor to see what is running, how much resources it is consuming, and quiting apps you don't think you need). However, that's a pretty minor issue.
Probably the best thing you could do is install a second hard disk for all your audio work, or if songs are getting complex with lot's of tracks, you can stream the audio from both discs. (Logic has a disk I/O meter that can tell you when your disk throughput is getting stressed ...)
Aditionally, MORE RAM!! More RAM equals more software instruments and samples playing faster, as the computer does not need to rely on Virtual memory as much. using Virtual memory is a big system slowdown problem, as the audio needs to "stream" of the hard disk, where it is temporarily stored. Virtual memory usually uses your primary hard disk, so this is another situation where a second disk for audio really helps.
Other than that, I personally don't do anything special anymore ... it ain't like a few years ago when scraping that extra little bit of resources together really helped! Of course, I'm using a quad G5 as my main production box, so there's a lot of horsepower there ... maybe that's why I'm such a glutton ...