Basically, with the E6300/6400 you want to run a DS3 or P5B htat can run skyhigh FSB to get your clockspeed way up there. Becuase of the low multi, you have to jack the FSB way up. If you are going with a more expensive board like a DQ6 or P5W DH then you can go with the E6600/6700 because these have the higher multiplier and you don't run the FSB up as high, but these boards seem to support the higher overall clocks.
Basically, do you want to have the best? E6300's are hitting 3.4-3.5Ghz Stable on good air cooling and with good ram and the right board. E6400's are hitting 3.5-3.6Ghz stable on good air and with good ram. With an E6600 people have been getting 3.6-3.9Ghz. The E6700 and X6800 have not had enough people playing with them to really show their potential, howegver the X6800 is the king, and would be best at OCing. The Multi on the E6300 is what is slowing it down. the allendales have less cache which means less heat and thus they can overclock like crazy. However, the E6600 is the only one that seems to be able to get into the upper 3Ghz with ease. However, considering that the E6300 is hitting about 100% overclock with instability issues, but easily doing ~70% OC stable, I can't see the point. It's just if your ego needs to have the E6600 or not.
The Asus P5W DH seems to be king of the E6600/6700/6800's, while the DS3 is king of the 6300/6400's.