We tested both GTX 470 and GTX 480 in our labs......
Publishdate:2010-11-05    
We tested both GTX 470 and GTX 480 in our labs running the 3DMark software inside a tower case with two fans of 120mm at 1600 RPM.
Room temperature was 28 degrees Celsius. The results were 68 degrees Celsius for the 470 and 75 degrees Celsius for the 480.
Installing MK-13 for your Fermi GTX 400 series card will be fine as long as the gaming software you're running does not exceed the average 3DMark temperatures.
Running test software such as Furmark would not be recommended since it runs more excrutiating and strenuous tests/instructions than any other existing software.