I run Yosemite on a 2007 20" with 4GB and an SSD without much issue for simple daily tasks. You can install up to 6GB (4+2) which would help as well, but the SSD had the greatest effect on usability. I bet it would feel insanely fast with an SSD in Snow Leopard... but I still stick with Yosemite for the iCloud integration aspect, regardless of the performance hit.