The 6300, 6400 and 6600 all overclock. But in order to do 3.2Ghz and more on the 6300 and 6400 chips you would need to purchase a 3rd party HSF and crank up the voltage.
If you get the 6600 you can use the default HSF to reach 3.2 and not to worry about any heating or voltage issues.
So.... get the 6300 or 6400, buy a $50 HSF and get to (or around) 6600 speeds. Or just buy a 6600, get 4MB cache, and pay only $50 over the 6400 w/$50 hsf.
The 6600 (if you overclock) is only $50 more than the 6400, and a better chip.
You starting with Zero, or upgrading wholesale?
Are you planning on overclocking? If so, to what extreme? Stock-Aftermarket air-water-phase change-LN2?
Depending on what you do, there are a large number of best answers to your question.
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