One of PimpRig's regular forum members has a very interesting project underway to see how far he can push an Opteron 146 using a custom machined cooling tube filled with Dry Ice. Pretty interested project worth a look. Here is a snip from his project thread:
"I along with the help from two fellow pimps alpha754293 and PhLaKz'R will be making a dry ice container to cool my cpu to some frigid temps. Some of you have seen phase change units and may ask why not use one of those. The reason I decided on a dry ice container rather than phase change is because of cost, power consumption, and cooling ability. Dry Ice can get the cpu colder than most single stage phase change units for a fraction of the price of a phase. Also if at a later date I decided I need to get it colder I could use LN2 (Liquid Nitrogen) which is -192C. That is colder than any cascade I have seen to date. I expect temps to be some where in the -40C to -60C range."