Joe - Replacing the heat sink won't fix this kind of problem. Unlike a desktop system were you could put in a larger unit there just isn't any room or alternative heat sinks that offer better cooling than what you already have now.
You need to clean off the old thermo grease and apply a fresh coat on the CPU & GPU units so heat is better able to dissipate into the heat sink. Make sure to use a good quality thermo grease. You will also need to clean out the heat sink fins of any dust as well. Don't forget to clean the fans and make sure they are running.
Lastly, you should look at your applications and any OS add-on's to make sure none are running when they shouldn't (a common cause of over heating).