The A2 DIMM slot does tend to be warmer than other locations. The newer 2008 Mac Pro rearranged the Risers so as to allow more air flow into the memory chamber.
For me, just a slight boost to the fans to 750-800 rpm was enough to bring them down below 70 C. However, even 73-81 C is below rated maximum temperature.
What make/brand of 3rd party memory? and of course, what happens if/when you put a lot of stress of your system memory?
What I did for awhile (I don't like to run any 3rd party background / startupitems that aren't 100% essential) was to tell Hardware Monitor to alert me when any DIMM got above a certain temperature (which I set to 75C and all my memory was running 65-68C normally).
I think I have bookmarked some places that publish FBDIMM specs but take a look at Barefeats.
http://www.barefeats.com/quad09.html
http://www.barefeats.com/octopro7.html
TEMPERATURE TESTS (2008)
We spawned 10 copies of DLT running for 10 minutes each. Below is the temperature history graph from Hardware Monitor. Notice it stays well below the 190 degree Farenheit "sweat" threshold. Though some FB-DIMMs run hotter than others, during the 10 minute stress test, the hottest module peaked at 174 deg F. Five of the eight modules remained below 155 deg F. The average for all eight was 159 deg F.
http://www.barefeats.com/harper12.html
FB-DIMMs are recommended to be equipped with sufficient thermal dissipation capabilities, to maintain a safe operating temperature on the FB-DIMM components, while with an air flow of 0.5 meters per second and at an ambient temperature of 25° C, during maximum memory bandwidth utilization.
The Mac Pro will ensure FB-DIMMs maintain a safe operating temperature by raising fan speeds to increase air flow, up to a maximum of 2 meters per second. If needed, the Mac Pro will also lower the maximum permitted memory bandwidth.
Note: To maintain a safe operating temperature at an ambient temperature of 35°C on FB-DIMMs using a JEDEC standard heat spreader requires an air-flow of approximately 3 to 6 meters per second. Therefore, designing adequate thermal dissipation capabilities on FB-DIMMs for the Mac Pro is strongly recommended to avoid raised fan speeds and lowered bandwidth during user operation.
In order for the Mac Pro to maintain a safe operating temperature of the FB-DIMMs, the AMBs for each FB-DIMM must provide a functional temperature sensor.
Note: The Mac Pro will run the fans at full speed when detecting FB-DIMM's without a functional temperature sensor, which will maximize cooling on the FB-DIMM's. While the full speed of the fans maximizes the preservation of thermal integrity, FB-DIMMs with inadequate heat sinking may still overheat depending on ambient temperature and system load. In this case, the Mac Pro may shut down the system to protect the FB-DIMM from possible thermal damage.
http://developer.apple.com/technotes/tn2006/tn2156.html
Mac Pro Dev Note: RAM Expansion
