Officially your mac can run 10.7 lion but not 10.8 mountain lion. Unofficially your mac can run mountain lion with the use of MLPostFactor, for more info head over to Hackerwaynes unsupported mac thread in the mountain lion forum. Either way, it shouldn't slow your macs performance. If you do intend to upgrade your mac then make sure you use a minimum of 2gb ram for your particular mac.