MSI RX2600XT Diamond OC Edition
 

The MSI RX2600XT Diamond OC Edition already comes OC'd out of the box, 850/1150Mhz for the core and memory respectively. Neither RivaTuner or Powerstrip allowed us to make any OCs that had a visible impact on performance. ATI Overdrive allowed a 6Mhz OC on the core and 21MHz on the memory.

http://www.viperlair.com/reviews/video/msi/rx2600xt/