OC'ing using D.O.T help!

hello, i have recently bought a msi nx6200ax from a store and i am kind of new to this overclocking feature.as it had no cooling fan so i installed one. it works perfectly fine but when i try to overclock this card using 'D.O.T', my games freezes and sometime the cpu hangs . my default settings are this .
2d clock : 350 mhz
4D clock : 250 mhz
Memory Clock : 600 mhz
these where my default settings . i want to overclock it and increase the game speed. is there anyway i can speed up this card? also i want to know about these d.o.t options ( private,sergeant,captain etc )