9.1 is going to be the newest one you can operate, though if you can't play DVD's, don't care about iTunes, or anything like that maybe tedj's advice on 8.6 should be followed.
MacBook 2GHz Intel Core 2 Duo w/ 2GB DDR2 RAM & 120GB SATA 5400RPM HDD
Canon Rebel XTI
Google Cr-48 Beta Laptop