Further to Gerard's reply, stay well clear of the common white paste you
see in TV's and amplifiers etc... totally useless for CPU, this stuff
tends to dry out rather quickly.

Use Artic Silver or similar paste as this tends not to dry out,
slightest of smears on one surface only is all that is required... not
like greasing a gear box on a lorry where you slap it on nice n' thick.

You can get thermal pads instead of paste (do not use both), thermal
pads are not as good though and makes the CPU run a few degree's
warmer.

The reason for using these is... if you look at the surface of the
heatsink under a microscope it'll look like the moon, the craters being
where no surface contact is made to the CPU, the idea is just to fill
these craters or imperfections... making believe the surface is nice n'
flat where the heat can pass easily.

davy