If you have the money, definately go with a q9550. You can hit 4ghz with this chip. I don't recommend q8200 or q8300, brcause their lack of cache really makes the harder to overclock. The q9400 and the q6600 will really be close in performance, so those 2 are just the cheaper alternative.
In my opinion, you should go with the q9550, or you will most likely reret it, especially since your upgrading from a fairly good cpu, the e8400