With the TES 1326 Infrared Thermometer literally in hand we found ourselves getting real-world temperatures from the products we test at load (3D clock speeds).
There are two places we pull temperatures from, the back of the card directly behind the core and if the card is dual-slot and has an exhaust point we also pull a temperature from there (as seen in the picture).
The 8400GS manages to get a fair bit warmer than the 8500GT but you could put this down to the fact that the 8500GT from MSI uses a larger fan which is able to cool the core more efficiently.
Pulling out the TES 1350A Sound Level Meter we find ourselves quickly yelling into the top of it to see how loud we can be.
After five minutes of that we get a bit more serious and place the device two CM away from the fan on the card to find the maximum noise level of the card when idle (2D mode) and in load (3D mode).
The bigger fan on the 8500GT is understandably a bit louder under full load when compared to the 8400GS.