The Q9450 is worth the extra money... for your application, a few reasons -- more cache, faster clock, cooler. But also SSE4, which some apps are now showing up compiled with the new instructions, this is worth 20-30% gain in many cases.

I have a QX6700 which I can clock to 2.4 GHz and a QX9650 which I can clock to 2.66 GHz ... though I haven't strictly compared them yet in video encoding, I can if you like me to. Let me know, the rigs are set up... let me know what encoding you would like to see.

Jack