The E6300/E6400 seem to overclock better than the e6600 etc. Though with the lower multipliers on the first two chips the speed seems to turn out roughly equal unless you go in for water cooling or better.
E.g.
e6400 @ 450 fsb with 8x multiplier = 3.6ghz
e6600 @ 400 fsb with 9x multiplier = 3.6ghz