there is no such thing as a 'safe' or 'unsafe' speed dude. Reading any kind of basics of overclocking even for graphics cards and you really wouldn't have to ask this question.
As I'm sure has been said you keep pushing it until you see artifacts on your screen then back it down.
To be specific, technically any kind of overclocking shortens the life span of a component, but the cooler you keep it, the longer it'll last.
loop 3dmark05's nature
if you can't do that, pm me and i'll give you the serial key for 3dmark05 that i'm using so that you can access the options and stuff
Illegal things such as serials, cracking, and hacking aren't allowed dude and you know that.