My 280x are doing ~330-335 sol/s (Claymore v12.5), temp. is about 60-65C, fans at 40-45%, undervolted a little bit from 1.256 to 1.15, drawing from the wall aprox. 220W per card. You can undervolt your 280x more, but I don't care about electricity bills too much, so 220w is ok for me.
To undervolt my 280x I have used VBE and ATIFlash, just google it, there are a lot of articales about downvolting.