A TEC (Thermo Electric Cooler) is a semiconductor device that uses the Peltier effect to make one side of a ceramic plate much colder than the other. They are very inefficent at cooling, you're typically looking at TEC producing twice as much heat as the actual device you're cooling. As a direct result you need to cool the TEC with decent watercooling.
They typically run off 12V (or 24V in some cases) and generally require moderate ampage between 10-20A. This is generally too much for your PC PSU so a standalone dedicated 12V supply is recommended.
What you actully need is a watercooling setup, plus special blocks and TECs for whatever you're cooling (DangerDen do kits). I would recommend a seperate PSU for a TEC.
You're probably looking at £100 alone for a TEC +waterblock and PSU. Then you've got to factor in the cost of the rest of the watercooling stuff on top of that. You've also got to bear in mind condensation as well.
CPU wise, phase is the way to go in my opinion but there is fun to be had with TECs on GPUs.
Jokester