Jaydee, don't make ur decision by logic, lets do it in mathmatics. Referring to this equation in this famous article:
http://www.amdmb.com/article-display...D=105&PageID=3
lets fill the same equation with the parameter of a typical watercooling sys:
Delta T= Q *(L/K), L means the thickness of the tube, and K means the conductive coefficient of the material of the tube, copper is 401 and aluminum is 237 and brass is only 109;
given the Q is up to 150Watt and thickness is .5mm;
then Delta T will be .000187 for copper and .000316 for aluminum, and .000688 for brass,
And then we must divide our initial answer by the tube area, which is up to hundreds times comparing to the die area of the Athlon 0.000117 m^2(=117 mm^2), given the total tube area for a single HC(15*15*5cm) is at least 13(tubes' No.)*3.5cm(atual tubes' width not 5cm)*15cm(tubes' length), result of the tube area is .0682m^2, up to over 600 times of the die area of the Athlon, so check the Delta T now, .00274c for copper and .00463c for aluminum and .001c for brass.
all of above is only math calculating, we still need to count some other factors, such as the temp diff of the liquid inlet and outlet of the rad, but as we know from BillA's rad test and others' article like Joe Citarella of overclockers, the temp diff between in and outlet is minors. so basically we can safely drive a conclusion , brass tube will not be beaten by alot.
Forgive me if I miscalculate above.