normal Q6600 user can get 3.0ghz-3.2ghz out of the chip which makes a nice cheap quad solution.
Q9400 has half the L2 cache as the 9550 but same specs other wise.
Q9550 has twice the L2 cache...is 45nm vs 65nm and can I see it clocked @ 3.8ghz or higher in most peoples specs...it also is also 1333mhz vs 1066MHz of the Q6600.
Personally Q6600 is great for price/performance on air expect anything from 3.0ghz to 3.4ghz but if you got a hole in your pocket the Q9550 is retarded and will clock nice....water cooling junkies get 4.2ghz-4.5ghz out of it...its crazy.