i did some wiking specifically: http://en.wikipedia.org/wiki/Johnson%E2 ... uist_noise
the thermal noise floor for 20mhz wide channel with 50ohms is -101dbm, and that happen to be very close to what the R52nh card reports most of the time.
-174dbm is there thermal part at 300kelvin
if one where to cool down the receiver amplifier to lets say 30kelvin one would push the noise floor to -111dbm a 10 fold improvement on you link budged. for sure not practical unless you looking for something to substituted 50dbi antenna.
but anywho my question is do you have to cool the antenna down too or just the pre amplifier in the radio card?