I used to have 2 x 280Xs in my rigs and I sold them on Ebay because they use SO MUCH electricity for a pretty low hash-rate. I had them core overclocked to 1150 and they did 17-18mh/s. Far better to use a 380. With a 380 at core 800, mem 1500 I get 20-21mh/s and using much less power. Or a 290/x at core 1100 mem 1125 gets me 30mh/s but uses a lot of power.

There was some discussion earlier about how flashing a BIOS would void warranty? Who would still have a 280X under warranty anyway? They are about 4 years old aren't they?