I would not recommend overclocking your CPU using the AMD Fan.
More importantly (if you really want to overclock).
The AMD XP2600+ is supposed to handle heat upto 75 degrees, but you will experience stability problems way before this.
What is your current CPU Temperature? Install ASUS Probe from your motherboard drivers CD.
NB - I have the same motherboard as you, check my system below.
[size=0.5]Full Aluminium ThermalTake Case
AMD Athlon XP 3200+ *400 FSB* Barton (2.2GHz)
ThermalTake Volcano 11+ Xaser Edition Air Cooling
1024MB Corsair TwinX XMS 3200 (Dual Channel 2x512) 2-3-2-6
256Mb XFX 7800 GS EXTREME
2x WD Raptors (10krpm 8MB Cache) 36.7GB using RAID -0
Video Logic SonicXplosion 5.1 Soundcard[/size]