The Arctic Cooling Super Silent heatsinks are a POS. The Thermal sensor does now work accuratley. We had a situation while testing the ARSS (arctic cooling super silent..) where our *STOCK* athlon xp 1900+ was hitting *55C* under idle temps with an ambient of just over 25C. The Motherboard alarm was constantly going off. The Thermal sensor does not work properly.
On another note, the FANS are very good. I love them, but I hate 'thermal fans' in general =\.
If you're going for decent performance and silent cooling (with an AMD cpu) look at the Thermalright AX-7 + a nice silent fan (Zalman 80mm fans, Vantec Stealth, Panaflo's, Papst...etc).
-JokerF15
EDIT: Didn't notice the '4 tc' meaning P4 heatsinks - The AX-478 is a pretty comparable heatsink to the AX-7, but some older batches of the AX-478 did have some problems with them not being welded/'glued' properly.