I have both those CPU's in various PC's in the house (my own and son's).
Both are pretty sweet and seem to handle all games just fine. I'd get a second hand Q6600 off ebay, give it a slight speed bump upto 3 to 3.2 GHZ and feel smug with your inexpensive £50 to £80 purchase.
Mild overclocking is a doddle and can be achieved by any chimp. Just keep an eye on the temps and look to improving your cpu cooling.
Once you overclock, you will never go back to running at stock again.
I've run both. Having overclocked both you're better off with the Q6600. "In my experience" the Q6600 can get up to clock speeds around 3ghz. Unfortunately I found the same to be true for the Q9550, for the same type of voltage bumps you don't get much. I was only able to get it up to 3.2 stable, anything more and it would fail out before I could get it to windows. At the same clock speed you'll get a little more out of the Q9550 but the difference is not noticeable. I vote Q6600 due to the performance per dollar, but if you already have both or if you're going for the better performing of the two the Q9550 wins. My two cents.