TheAM2 CPU's don't seem to hungry for voltage, so you should try to get the best OC at default voltage first.
Other things that effect the OCing.
1. CPU temperature, upping the voltage increases tempes... keep the full load temp @ or below 50c
2. Memory... lots to cover there. As you increase speed of the CPU the speed of the memory increases and things to do to counter that.
a. The RAMs controler uses a multiplyer of 5, reduce to 4x for a CPU bus of up to 260MHz, beyond that drop to a 3x
b. RAM it self, more voltage can get higher speeds, do some googling to find your RAMs known safe level of voltage.
c. RAM timings, they may be adjusted to loosten the settings to allow for higher speeds.
d. Default RAM speed. Your DDR2 800 could be set to a lower default speed, that gives the RAM's bus speed room to increase as the CPU bus increases.
A good guide for the pre AM2 MBs, simular and very relavant to AM2 OCing concept