The Q6600 and the Q9550 are both two processors created with different manufacturing processes. Which one is supported depends on your motherboard, newer models usually support both. As for the power supply issue, that depends on the rest of your hardware, mainly the video card. Obviously the faster or number of components there are, the more power is needed. In terms of which aftermarket cooler you should use, that depends. Many coolers advertise themselves as quiet. The ARCTIC COOLING Freezer 7 Pro
is very quiet. SilentPC
has a list of heatsinks that are rated based on the amount of sound they make. They give the XIGMATEK HDT-S1283 120mm Rifle CPU Cooler
one of their highest rankings, but it is quite large.