There is a lot of good articles about peltiers on the front page of Overclockers.com in the tips and reviews sections. I suggest reading through there about them.
The reason that most CPU's will overcome an 80 watt peltier is this. The pletier uses electricity to move heat from one surface to the other. This creates a cold side and a hot side. When a CPU, like the modern ones which create about 120watts (i think this is an approximate figure, correct me if I'm wrong) of heat energy, is mated to an 80 watt peltier, well, more heat is being generated against the cold side of the peltier than can be transfered to the hot side of the peltier through the elctrical process. The CPU cannot be properly cooled.
Happy reading