a 6x6x2 heatercore is a pretty good for pc watercooling... the only thing that will really beat it is either a bigger heatercore.. or a heatercore that has a higher fpi (fins per inch), or just having 2 heatercores in parallel
. either way you might get 1 or 2 degrees difference at best that just using a single 6x6x2 with an xp at around 2 volts... so a 6x6x2 will do fine as long as you use a shroud with a 120mm fan (unless you are getting a 172mm rotron fan
) and have the fan sucking air from the rad...