At the moment I have a Scythe Ninja passively cooling my E6600 @ 3.0GHz inside my HTPC case. With two instances of CPUBurn (for 100% load) for over 30 minutes, Core Temps measures the highest temp at 58C. Of course when the weather starts getting warmer, the temp will most likely increase.
There is also an all copper version of the Scythe Ninja now. Since copper is a better conductor than aluminum, I would guess my the max temp would drop 3C or 4C.
However, the fan that comes with the Scythe is pretty quiet. I don't use it because it is a tight fit in my case and the wires coming out the back of the hard drives simply gets in the way.