If you have at least 1GB of memory: Snow Leopard (OS 10.6).
If you have at least 2GB of memory: Lion (OS 10.7) or Mountain Lion (OS 10.8).
Upgrading to Mountain Lion (OS 10.8) also depends on the year your Mac was made.
supportDOTappleDOTcom/kb/ht5444 (replace each “DOT” with “.”)
You will need OS 10.6.8 to upgrade to Lion or Mountain Lion.
Both Lion and Mountain Lion are no longer directly available from the App Store. You need to go to the Online Apple Store to get a “content code” in order to install Lion or Mountain Lion from the App Store.
Lion: storeDOTappleDOTcom/us/product/D6106Z/A/os-x-lion
Mountain Lion: storeDOTappleDOTcom/us/product/D6377Z/A/os-x-mountain-lion
