While Snow Leopard is content running with 1 GB, Mountain Lion needs at least 4 GB. So for older machines, a memory upgrade will probably be required. Memory specs vary a lot, and machines manufactured prior to 2007 might not support 4 GB. Memory supplier Crucial.com offers a free tool for determining which type of memory is needed and how much a particular machine will support. We recommend taking a trip there to see what's needed; you can use the tool even if you're not ordering.