From everything I have read the E6420 will OC to atleast 3.2+ and most I have seen are pushing 3.5 with an aftermarket cpu cooler (Tuniq or Artic Freezer Pro 7). Not to mention it's $50 cheaper than the E6600. I would say get the E6420 due to the sheer cost difference as the performance will be practically the same.
I was going to get the E6600 until I started reading about how well the E6420 OC's and noticed that it is cheaper!
You'd be sort of lucky to get the e6320 to around 3.6.....I had a hard time getting mine to 3.6, and it was still unstable. The e6600 would get to 3.6 easier.
Would you say the e6320 is up there with the e6420? Don't they have the same overclocking potential? I think maybe the e6320, might need a little more juice though, to get pretty high.
Well originally the E6300 and E6400 were 40 bucks apart...and the extra 450mhz or so you could get out of them wasn't worth the cash. However now that it's just around 20 bucks I believe the extra multi is worth it. Still though they are both excellent choices because of how far they can go.