You can overclock it as much as thermal head room will allow aswell as being within safe voltage limits.
From what I've seen the 70c range is pretty acceptable for those chips and 1.25v / 1.3v in higher end overclocking (water cooling).
Noone will be able to tell you voltage and frequency with 100% certainty since each chip comes off the line differently, everyone will have a different result to some degree.
Try to look for a few guides that do it in steps, raising clock then voltage as needed to maintain stability.
Make sure to check your motherboard and PSU are able to handle an overclock too.