I have a Q6600 right now, its running at stock speed currently. But i guess the real question is, is money an issue for you? Im a poor college student and as much as i would like to have a Q9550S, it really would not make any sense. One thing i have stood up for before, and will push again is, "Dont put money in an outdated platform". This is what has pushed me to stick with the times.
At this point, the Q6600 is like 4 years old, the Q9xxx series is like 2 years old. The difference between the 2 is not going to be huge. So unless you have money burning a hole in your pocket. I say no.
Save your money for beer, girls, food, or some other upgrade that will last longer than that CPU. If i was going to buy any hardware right now it would be an i5 platform. Because as much as i would like a Q9550S, with some DDR3 and a P45/X45 chipset, it wont make my computer any faster (or not very much faster).
Back in the day, i had my Athlon X2 3800 on a MSI nvidia NF4 SLI motherboard. It was not what i wanted but its what i had the money for at the time. I had always dreamed of getting an Opty 165 and the Asus A8N-SLI Deluxe. One day i had the money and made the switch over, and it really was not that much faster. I could have saved my money for a Conroe platform when they first came out.
I realize this is probably more information than your looking for but, if you want it and you have the money go for it. Nothing we say is going to change that. I mean who doesnt want 12MB of L2 Cache. But your not going to see HUGE changes.