This is just a very rough estimate, as my need for Windows HD space will go down once Steam for Mac releases.
I'm thinking between 70-100GB for Windows, and the rest for OS X. I'm not willing to go without 250GB+ HD space). For earlier Mac operating systems, please use microKORG Sound Editor 1.01. This software requires that your Mac be running 10.6.8 or newer.
This software is Intel-Mac and OS 10.7 (Lion) compatible.