Well, by my calculations with the help of your post I've come to this. The 680 is 256- bit and the memory clock is 6000, if the memory clock should be increased 1000mhz with each GB with 128- bit cards then with 256-bit cards it would only have to be increased with 500mhz per GB. This would mean that since the 770 has a memory clock of 7000mhz it would be able to use all of the 4gbs right?