The numbers out there are about even as to overclocking.
In comparison the e6300 and e 6400 clock a little higher when comparing to the original fsb of each individual CPU.
The E6600 clocks a little higher frequency wise but you’ll hit a fsb strap earlier with the 6600.
If you lower the multiplier down you in actually end up overclocking the NBCC
NBCC = multiplier default / Multiplier set x FSBhttp://www.bleedinedge.com/forum/showthread.php?t=23803
All I can offer is what I have so far on my E6600http://img133.imageshack.us/img133/5108/10122006075740oo9.th.png
Maybe someone can post a similar screenshot with an E6400.
You’ll be able to see some comparisons searching thg and other forums.