Personally I fail to see why you would need 6GB of graphics memory being tied to one GPU, the regular 3GB already should be aby far sufficient enough. That didn't stop Sapphire to release a 7970 6GB TOXIC.

The card boasts 6GB of the latest GDDR5 memory, a powerful 10-phase power supply known as the Lethal Power.

By default, the HD 7970 6GB TOXIC Edition runs at 1050 MHz and with PowerTune Dynamic Boosts rises to 1100 MHz on the engine with the memory clock at 6000 MHz effective. By pressing the new SAPPHIRE exclusive Lethal Boost Button, the beast is unleashed, boosting the core clock speed to 1100 MHz with PowerTune Dynamic Boost to an industry first of 1200 MHz on the engine while the memory is further overclocked to 6400 MHz effective.