I'm running OSX 10.7.5 on a macbook air with 1.7 GHz Intel Core i5 and 4 GB 1333 MGz DDR3. It's kind of an early version of the air. It's started to slow some, I've had problems with the fan running constantly and overheating but no crashes. I checked the CPU monitor and managed to get the fan to mellow by shutting some background applications. I'd like this computer to last so I'm cleaning things up and updating some things. Thinking of upgrading my OSX so I can install more recent antivirus software and generally improve performance. But I don't know if I have the computing power or if upgrades above my current OSX are less efficient. What would you recommend for OSX updates? Or should I stick with 10.7.5 ? I would like any relevant upgrades to be free. Thanks!