A Q9550 will be a nice upgrade
IF your board supports it. Gaming will see a pretty big performance gain with the huge 12MB of L2 Cache it really helps to push High End GPUS nicely also overclocks very well too but this all depends on 1 thing
your motherboard.
The Q9550 will be very close to an I5 750 in gaming and with a good overclock can match and exceed performance of a stock I5 750
http://www.anandtech.com/bench/Product/50?vs=109
And will also perform about the same or slightly better clock for clock with a Phenom II 965BE.
http://www.anandtech.com/bench/Product/50?vs=102
So yea if your board supports it and you looking for a nice upgrade the Q9550 is a great deal and if you near a MicroCenter and can get one for $179.99 you can really not beat that for an upgrade. But anything lower than a Q9550 is really a pointless upgrade and you would be much better off sticking with the Q6600 and overclocking it.