What is a stable clock speed for my HD 4850 for gaming?

Hi guys,

I want to OC my Palit HD 4850 for gaming:
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

Normal clock speeds when gaming are 625/993. AMD control center only lets me go up to 675/1050, but I use Sapphire TriXX to see if I can push further.

I keep trying to go 675/1050, but whenever I do, my screen locks up and in 10 seconds the screen blinks and says the driver has crashed but has been recovered; and this happens a lot of times.

I don't think temp is the problem, I'm getting high 50's-low 60's with my fan speed at 6% (automatic) when playing SWTOR according to TriXX, so is there something I'm not doing right?

Thanks!