I never topped my retail hsf out...I just ran up the fsb until the cpu got to hot under load for my standards... The problem with the retail sink for like m,y 1.6 and 1.8's was that it only runs at 2700rpm or less and that is not really enough once you get past 2.4ghz. The temp at 2.4ghz with my 1.8 and it was like mid to upper 50's on an asus mobo which I know records lower temps and likely would have been 66c on my epox. My alpha at speed listed down below tops out at 50c on my epox mobo after intense cpu burning. This temps would be in the mid 40's on an asus mobo I am sure....
Make sure you use AS3 properly applied...It takes about 2-4 days to cure and temps over that time can drop up to 2-4c as well in my experience