The change from 10.4 Tiger on 10.6 Snow Leopard was a quantum leap! The calculator is felt with the SL twice as fast as before. The installation was a poem. I do not have a single program that does not run on SL. (The oldest is from 2002 !!!) The new OS saves you smooth away a new computer. My old box is launched to new heights, since the SL is on the plate. As the differences turn out to 10.5, I can not judge, because I did not have this version. (The Tiger was also damn good.) Allen, who still have on the board, I can only recommend that you switch to the new, current 10.6 Tiger.