The stock HSF works fine, but you might consider getting a custom (preferably a larger one) especially if this person is uncomfortable with opening the case up and vacuuming the dust buildup from the stock a64 cooler. I have a A64 clawhammer and my load temps are ~50c which is well within the operational range. Athlon64s are rated to work up to 70c! Please note that it probably will be a good idea to vacuum the dust off the stock HSF once per year at least. The heatsink grill plates are positioned very close to eachother, and dust can get in between them and greatly impede airflow. After running my a64 clawhammer for 1.5years (24/7 operation, and in dusty enviornments) i had so much dust inside my heatsink that i could not see the metal when looking head on. The dust was so bad that my idle temps were 60c and load temps were over 70c! The exhaust from my two casefans was HOTTER than my old antec true430, and that thing ran pretty hot! These extreme temps were causing the core to ramp its clockspeed down (or shut of temporarily, im not sure of the specifics), but it was causing noticible slowdowns in XP (system would just seem to hang for awhile) and some programs were reporting memory problems. Prime95 was failing immediately and memtest86 causes my system to shutdown!
My system was probably running under these conditions for 2-3months, i vacuumed the dust, and now its w