Hi, Its a newbie question. I apologize. Does OS X need authentication serial codes like Windows when installing? Also, can one upgrade disc be used on two different macs? Trying to find the most cost-efficient way to upgrade from 10.4 (Tiger) to 10.6 (Snow Leopard). Found that going from 10.4 ---> 10.5 (upgrade disc) ---> 10.6 (upgrade disc) would be best. But was wondering if I purchase 10.5 upgrade disc from ebay, if it could be used without any forseeable problems. Thanks for the help.