There is no point to downclocking the CPU - the G3 upgrade is independant of the existing CPU itself (that is, it is not used), nor are you going to "cook" the existing 601 CPU (it hardly runs hot anyway!). The Newertech upgrade should also let you set the multiplier and/or bus speed to a rate you are comfortable with, such as 35 or 40Mhz - it also control the bus speed independant of the clock generator on the motherboard itself.

JB