You could lower your bus speed. Another possibility is to lower the multiplier (but I"m not sure if that's allowed your with that CPU or not). From there you can lower the voltage, more than likely.
On my athlon 64 3000, I"m running at 1.36v. Not sure if that's stock or not....DFI's tool implies that 1.5v is default.
I think I use to undervolt my celeron 300 in the old days (and still OCed it).