Hey there,
Not really any more tips. You are lucky to get 70mhz increase on the clock at all.
Those cards aren't designed to OC, and come with very basic cooling. There is no point in pushing that card beyond 620mhz. It will not make any difference to your games really.
Anyway, when you PC starts crashing it points that the OC is not stable. Lower the clockspeed until it stops crashing. That's your limit.