I don't t personally think it's worth it unless you're hurting for performance on things like video encoding, which might benefit from SSE4.1. For games and other everyday use, you're not really going to see any noticeable difference, and 4GHz+ is at the high end for Q9550 overclocking too (the FSB is likely to be a problem). You may only be able to get 3.8-4.0GHz.
If I already had a Q6600 (particularly G0) I wouldn't be looking at a Q9. SSE4.1 may help someday, but IIRC it'll require coding optimizations to be used. When the coding optimizations are included in your transcoders, then look and see.
A Q9 isn't likely to be "better enough" when you already have a Q6. I have an E6400, and I am eyeing Q9450s for a deal.