Thaisnang :
How much OC you can do also depends on your PSU. But if you have a really high quality PSU then you can hit upto around 4 GHz. You can go beyond that but I'll recommend a better cooler for that.
So, what 650W PSU (Because i'll buy a Sapphire Radeon 290 Tri-x OC) and CPU cooler do you recommend?