Silver thermal compounds are the best mainly because silver has one of the best thermal conductivity.
Thermal grease is not that good, you will notice a difference jumping to silver compounds. I lost 6C by using AS3 instead of the thermal grease supplied with Zalman.
I also read a different silver based compound that actually did better than AS3 but the difference was tiny, about 1C under certain conditions.
So get silver , its the way to go.
Thanks for the pointer. I went with the silver (AS3), but also changed the heat sink at the same time. So I don't know if changing the thermal grease had any effect. However, the temps are low, so at least nothing went wrong.