i use retail hsf on my 1700+, looking at the MBM log in about the lats 32 hours or so my fan has ranged from 4440 to 4687 rpms. have you tried plugging the fan into another fan header on the mobo?
if you have the guts and the know-how, try removing the yellow wire from the 3-pin block so that you can power the fan using a molex->3wire connector but still connect the yellow wire to your mobo for a tach reading.
i cant believe i said know-how