[BrianG]

The problem with just using paste is that the board can move around inside the case a little. And with thermal grease (of any kind), movement of the mating surfaces will reduce its effectiveness. In your computer example, you state that the sinks are also bolted - that keeps the mating surfaces consistent. And just think what would happen if dirt gets in there and starts mixing with the grease as it moves around! Ugg. I agree with ZPB on this one - thermal epoxy is all but required.