I think the MDD has a limit of 2GB, but OS 9 can only address up to 1.5GB in either case.

Which is kinda interesting. That's another thing to add to the Mac OS 9 wishlist: Support for the full 2GB of RAM.

Another interesting fact is that 9 forces VM off once you hit 1GB. While you really shouldn't need VM with that much RAM, I have heard that having it on results in a better a job of flushing memory when closing an application.