Womp,
To fill in what others have said, OS 9.x was the last release of the Mac "Classic" operating system, which was not based on Unix.
The 10.x series is based on Next/FreeBSD/Mach. The newest version, 10.3, plays quite nicely in Windows and/or Linux environments.
For just fooling around and learning, I don't think you need 512 Mb of ram, 128 would do. Be warned that if you run any "productivity" apps like Office or Photoshop, you will want the additional RAM.