Actually, AMD recommends you DO NOT use the standard, silicone-zinc oxide paste that the industry has been using for years (like the Radio Shack stuff). The T-birds run extremely hot and the paste does not hold up over a long period of time. They recommend sticking with the "change-phase" material (thermal pad) that comes with the heatsink. This stuff can be identified by their bubble-gum like texture. They don't work as well as regular thermal paste, but they don't wear out either. They may appear thick, but it quickly melts and thins out with pressure and heat. All this is documented over at AMD's site and is available for reading, but you'll have to surf and find the right .pdf file.
Personally, I use silver grease, either from CircuitWorks or Artic Silver.