mac os x minimum requirements?

someone i know wants me to install mac os x her old ibook (the kind that looks like a clam and is colored like the old imacs) so she can run some newer programs that dont work on os 9. i was just wondering if os x has some minimum requirements because her ibook is pretty old.

I don't know about a minimum, but a lof of my Mac friends recommend 512MB for an enjoyable experience.