Exactly, you need Thermal Heatsink Compound. The better stuff out is just like the last guy said, Artic Silver Thermal Compound. Grease will heat up and spread much too easy and not allow for a good contact, plus grease runs the chance of melting too far and making contact other parts. Messy.
T-bird 900 @ 1050mhz, max temp running SETI@home is 46c - 48c, idle at 34c. Have a fan in front, one in back, and using a Global Win FOP32 with Radio Shack thermal paste. Trying Artic silver later today to see it I can get to 1100mhz.