At standard the performance is very similar but the price difference puts the Q6600 ahead.
Clocked at the same speed (small OC on Q6600 or small underclock on Q9550), the Q9550 is about 10% faster - not noticeable unless you are benchmarking.
Q9550 has a 16% advantage on clock speed. Added to a 10% IPC increase are they so similar with the OP not overclocking?
Still not quite worth the premium however especially with you using only a GTX 460. Q9550 is the faster CPU but if the Q6600 is significantly cheaper go with that.