Q6600 is a 65nm process which means that it will use more voltage and produce more heat than the Q9550 which is a 45nm process. If you don't plan on overclocking, the Q9550 will be better for you because of the higher clock speed. I can see the difference in games from having lower clock speeds. In DODS with my Q6600 at stock 2.4 ghz, i get an average of 50 fps. But when I overclocked it to 3.0 ghz, my fps jumped to 80+ depending on the map. It depends on what you're going to do with the processor but for games, the higher the clock speed, the better.