What samh004 is getting at is that if you don't have any Classic applications you use, you can delete everything--OSX doesn't require Classic to be around to operate properly (heck, it's officially dead as of Leopard anyway).
If you actually do need/want Classic because of some old application, there are theoretically a few things you can get rid of, but frankly it's probably not worth it--the handful of extensions you can disable aren't likely to add up to much more than a few megabytes of disk space and probably won't make an appreciable difference in performance.
[Edit: Or you can go with trainguy77's theory (posted while I was typing my own), and find out just how much you can delete and still have it work. I used to do this years ago as well, but that was back when I was running a 250Mhz computer with a whopping 48MB of RAM.]