When assembling a new system or performing a CPU upgrade, it's important to remember to use heat sink compound between the heat sink and the CPU. This improves heat transfer dramatically and this keeps the processor running smoothly. You must make sure not to use too much or too little of this compound. Too much and it will ooze out and make a big mess. Too little and good contact will not be made.

Here's a trick I figured out to getting this right:
Spread a thin amount on the processor and then clamp the heat sink onto it the way it will be during operation. Then lift the heat sink straight off the processor -- don't slide it, just pull it straight off (surface tension from the compound may make this slightly difficult). Look at the surface of the processor. Where the sink was making good contact, you will see a stippled effect in the compound, like a stucco ceiling. Where the sink wasn't making good contact (because the bottom of the heat sink is not perfectly flat) the compound will still be smooth. Add a li ttle more compound to these areas and repeat the process until you get good contact over the whole surface. Note that some processors have a raised area in the middle -- you can't make contact over the whole surface area of these CPUs and you're not supposed to, so don't try! :^)

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