im confused. I have had a look at an Athlon 64 3400 and it showed a base setting of 2.4 GHz? a LOT of retailers have been notorious for supplying the wrong AMD processor as they get mixed up with model codes, it might be worth checking out. you might have an Athlon 64 3200+ which has approximatly a 2.2 GHz base setting...
As far as clocking is concerned you need to specify what cooling and also other hardware such as the motherboard ect.. so that anyone can get a genereal idea of how far and how stable the system can be taken. Most AMDs are remarkably stable at higher clock speeds. You can buy an Athlon 64 2800+ and clock it to 2.4 GHz; with sufficient cooling you have an instant Athlon 64 3200 for Athlon 64 2800 money :) i couldnt say if the setting you said is stable or not, at a guess yes as its not much of an increase but at the end of the day it is your system and i dont want to give any false sense of security towards you frying anything!