Are you having stability problems?
I have seen Palominos run quite fine at 64-65C full load, and Tbirds run stable at 70 full load.
Athlons are spec'ed up to 90C, so even at 64 there's still headroom left for the error in reading on the thermal diode.
Still 64C is a bit on the high side and in addition to Boaz suggestion you could put up a "table fan" next to it to cool it down further.
Anyway.. downclocking woul also reduce the heat, no doubt about that. The cpu will, at alower clock, draw less watt, and thus dissipate less heat. So if you can live with the performance decrease (and yes it will be signifigant) that option would give you some, although not much.
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