i think more then the motherboard it depends on what stepping of cpu you have. i have seen some 660's on 680i's that run out of steam at 3.0 ghz. before they need a large voltage bump. you should be able to bank on 3.2-3.4 running at 1.45v depends on how high you want to go with your voltage.
a stock h.s.f will give you a good o.c depending on airflow in your case and if the fan runs at full speed. do you have these parts already or just thinking of getting them? from what i have been seeing people are geeting as good o.c's at lower volts with a 6400. with the money you save, get a better h.s + fan
I Already bought them!!! but dont worry lol i bought thos parts in Hong Kong,,so i bought the processor at the price of a 6300 in the US !!!so its fine!!lol
so anyway i forgot to mension also that i didnt installed them yet.. im waiting for Vista Ultimate..(which i already ordered) .
and to go back the the subject, well i leave my case open all the time .. so there is a pretty good airflow...(i dust my case every2 days. so it wont accumulate to much .) and by the way.. with my new config.. i also have the ge force8800GTX so ...i think maybe its better to get a cooler...dont you??
no i cooled the cpu and and the chipset. gpu's fans are usually pretty close too silent and i didnt want to add the extra heat to the loop. with watercooling a gpu the extra o.c gained by it is usually pretty small too.