overclocking AMD X2 6400+

[NoControl666]

i tried overclocking my cpu which is a AMD X2 6400+ i only got it up to 3361MHz..my cpu frequency is at 210 if i try goin at 211 it fails to boot..i wish to overclock it a bit more to about 3.6 or 3.8...can some1 tell me y its not allowing me to past the frequency of 210?thx.

motherboard: ASUS M2N-SLI
cpu: AMD Athlon(tm) 64 X2 Dual Core Processor 6400+