I have 2 volcano 6cu+ heatsinks and they are quite loud and in my opinion they dont cool too well. got one collecting dust, and the other is on a xp1700 non-overclocked, it runs about 48 degrees idle with a 80mm fan mounted to the side of the case blowing fresh cool air at it, the case also has a 2 fan psu, 1 80mm exhaust fan at the rear, and 1 80mm intake fan at the front.
I would invest in a pretty good heatsink if you plan to overclock. Personally I like the Thermalright SK7's but they are quite large and do not come with a fan. In a way its nice, since you can choose whichever fan you like to use with it.You can use 60mm or 80mm fans with it.
Like Arminiusc said less rpm and more cfm will be quieter.
My 2400 is running 2200mhz at 39 degress chewing on Lifemapper and chessbrain at the moment.