Athlon II 620 x4 would not overclock past 3.2GHz

Hi all,
I have my athlon 620 cpu, it has been working for just over two years in my computer at clock speed of 3.2GHz, (stock is 2.6GHz), I want to be able to get more out of it, ive seen videos on youtube and read about others with this same CPU running at 3.6 and even 3.8GHz stable w/o water cooling.

when I try to overclock it past 3.2 I am able to go up to 3.4GHz but will freeze and restart soon. any higher and the PC will not even load windows.
here are my specs:

AMD ATHLON II X4 620 @ 3.2GHz
MSI 770-C45
8gb Nanya DDR3 1333MHz
320GB HARD DRIVE SATA II
300GB Hard drive SATA II
X24 DVD-RW SATA II
650WATTS PSU
XFX HD 5850
Windows 7 HP 64bit

there is a guy on youtube who has the same cpu and motherboard as me is able to get 3.8GHz stable.
could the problem be cause of the RAM

they are all Nanya RAM sticks,
three are 2GB 1333MHz and one is 2GB 1800MHz, under clocked to 1333MHZ, but even when I had just the 2 x 2GB 1333MHz RAM I was not able to overclock any higher,

any recommendations, I may consider getting new ram sticks.

All help appreciated, thank you.