Remember that overclocking is not a guarantee--the only thing that is guaranteed is stock speeds. In general the Q6600 is an excellent chip to overclock. It's also 2.5 years old using 65nm technology. I'd personally lean to the Q9400, all other things being equal.
Ive had the Q6600 and it is a great oc potential with high multiplier. However, I've since sold it and went with the q9300. The Q9400 should be better overall if you compare Q6600 and Q9400 @ same oc speed because the Q9400 is 45nm, and has a natural faster rated fsb. Don't worry about the Q6600 8MB Cache as being a huge advantage because the other stuff more than makes up for it - although gaming may produce different performance figures based on your gpu and the actual game itself.
Big advtange would be the 12mb (9550) cache over the 6mb (9400). They are pretty much the same in most other respects (yorkfield).
Remember that these have low multipliers, so you'll need a mobo that can get pretty good fsb. My personal fav right now is the Biostar TPOWER I45 - for reference, I have my Q9300 OC'd to 3.7Ghz with a 494fsb (roughly 2000Mhz Fsb) with a vcore of 1.28 at load (load temps are a little high at 66C, but that is about to change when I go to liquid) - high fsb potential (known to reach 600), full atx crossfire, and less than $140.
The board could go higher and I might try for 4.0 when I get my liquid cooling setup - but I'm thinking I'm pretty darn near the ceiling for these chips.
Forgot to answer your question: IMHO - It's only worth the +$50-$75 more if you do a lot of video editing/coding stuff. For today's games, the most important factor is the GPU. You'll see most noticable performance gain around 3-4 MB cache over 2 or less, but anything beyond 6-8MB cache and it's purely crunching numbers on a full load basis.
Nice OC - thats the great thing about the q9550 has a higher multiplier than a q9300 - Im limited to 7.5.
Crazy voltages arthurh - what mobo are you using and I am assume you are using liquid cooling? what is your graphics card also?
ASUS Striker II Exreme, 2 X 2G of OCZ DDR3 2000 at 1:1 and yes I am W/C'n. I use an Apogee CPU water block and home made cooling after that. 2 X BFG 8800GTS (g92) 512 OC'd in SLI. Used Rivatuner.
Yes your right about those voltages, but when you OC this high with a locked multi CPU..... I have the capability to run chilled water (33~35F) thou this OC was with 50F. My load temps at this temp are ~44-45C running OCCT or Prime95. But since I had to use such high Vc to achieve this OC do not leave it there. I don't have a replacement CPU.
I just wanted to see how much I could OC this with what I have. I usually run @ 3.83Ghz and play FSX with 1 GPU. Can do that at 1.3625 Vc as set in CMOS. Get some Vdroop when running PC but its rock solid stable at that set-up.