I'm not too familiar with the E8400 but the first thing I see is the multiplier. Normally with Intel Core 2 CPUs its 9, and drops to 6 under speedstep, so check your bios to see if it's set to 9 normally.
Also, as far as what test you want to use depends on what you want to test. Typically errors show up in an hour or so but whenever I do an overclocking build I do 8-24+ hours (I ran my current build for 72 hours, overkill but I went away for the weekend unexpectedly)
A CPU below 50 with a cold heatsink means that everything's working properly and that heat's getting the hell out quickly