These are the core/memory settings that have worked for me for the various 280x s ive purchased.
1085/1500
1035/1500
1050/1500
My cards' performance have varied. The older cards I got could hit 750 kh/s while all the newer ones can only get around 725kh/s.
Thats nice.. I am running mine at 1049/1500 pushing 732 and another one is stable only at 1020 and pushing only 702 khs.. but as long as I even work on that PC, its very good, because hash rates dont fell much and PC is perfectly controlable.. way better than 7950 on I200 then you cant do anything on such PC