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Worth it? That's for you to decide.
It will be FSB limited for OCing (due to it's 7.5x multiplier), and ddr3 is ridiculous overkill for this chip. It will run cooler, but may not clock as high as a Q6600, depending on cooling, board limits, etc. considering it takes 480FSB to hit 3.6 on the Q9300 vs. 400FSB with the Q6600. Also, the 9300 is going to have less cache, but supports SSE4. If that's something that will benefit you (encoding, etc.) then it may be worth it.
I do encode alot I like the temps from the 45 nm but can't afford q9450 and q6600 runs to hot on air and even on my wc rig for the most part. Seems like q9300 ist best option and with the board and ram I have 500 FSB is not out of reach@ 7 multi since half multi never clock well. 3.5 or so at a reasonable temps is kinds what I'm after. Now if I encode alot would I benicia from 2x the cache? Maybe save for the q9450?
FWIW, the Q9300 on average is roughly 7% faster than the Q6600 at matched speeds, runs cooler and will draw less power than the Q6600. As far as SSE4, yes it will improve performance in apps that use it but there are few programs out there that support SSE4that I know of such as VirtualDub and TMPGEnc 4.0 XPress.
@RayDaddy the extra cache right now from the benches that I have seen, performance increases have been minimal at best.