I have 3 PowerColor Radeon R9 280X TurboDuo OC. Nice and quiet cards, a bit wormer than gigabyte. Running around 710 khash@1,09V. Not the best overclockers, low ASIC reads (60% ish), so to reach 750 khash, You'll need around 1,15V and 1080 MHz core speed. Still the price is very good. If you dont care about electricity costs, powercolor is not a bad chioce. TBH, I would look for Sapphire Dual X (11221-00-20G).
now is hard to find Sapphire Dual X here
they say that those R9 280X are good at Watt/MH, is there some cheaper card with a similar hashrate, but poor in Watt/MH?
there is good thread about 280X https://bitcointalk.org/index.php?topic=470496.new#new