Raise the core up 10 mhz at a time. Run a game or 3D mark. If its ok then raise another 10. When u get to 340 stop.
Now raise the memory 10 at a time till the screen shows white spots or streaks when playing a game. When it does this at say 660 or so, back down 10mhz.
That would probaby be your max OC. There is no "safe" number. U just go up till the card starts malfunctioining then back down a bit.
If ur really scared just raise the core by 20 and the memory by 20 and leave it . Seems like all GF4600 can do that easily.