2006 iMac5,1 Upgrade from 10.6.8 to a faster OS
Solution 1:
According to EveryMac, the maximum OS X for that machine is 10.7.5 Lion, which is fine except that it is not free and the web browsers for it are obsolete. Safari will frequently fail due to invalid certificates. The best browser you can get is Firefox ESR 45.9.0.