We have a mid 2007 iMac. We have about 150GB free and running snow leopard. It's very slow and we're trying to boost it's performance. It has 3GB ram. I was thinking of getting el capitan as we want updates anti virus support. Do you recommend getting el capitan and if you don't, what is the best free anti virus for snow leopard? and what can we do to boost performance without having to spend money?