You could have saved yourself as much or more heat/energy by locking the card affinity to a single core, and still had a usable computer.
While setting the process affinity to a single core definitely help, it does not lower my system wattage to the underclocked levels. When I went back to my overclocked settings and set both phoenix processes to one core the watt used by the system went down to ~465w.
With the underclocked cpu, putting an 80mm fan on the hotter video card and setting both phoenix processes to one core I am down to ~420w!
Note that the fan trick is another power saver. By lowering one of the cards down from ~75c to ~70c I am saving 5 watts.