Every card is different so we can't say overclock to xxxMhz Core and xxxxMhz Shader, because it might not be stable for your card.
Personaly I overclock as much as my card will let me, but making sure the temps are fine and that its stable.
I run the Crysis benchmark x 20 to test for stability and max temps ( Run GPU-Z in the background and log temps)
If you don't have Crysis, then you could use Furmark to test for stability and max temps which you can get here: http://www.softpedia.com/get/System/Benchmarks/FurMark.shtml
And GPU-Z to monitor temps: http://www.techpowerup.com/downloads/1781/TechPowerUp_GPU-Z_v0.4.0.html
I take it you already have EVGA precision?