The 6670 is alot faster. However, if you can spend another $20, the 6790 is quite a bit faster than both and the core is actually related to the much more expensive 6800 series and not the 6670. The 6650 and 6670 are actually badge engineered previous generation 5670 (redwood) cores with a different BIOS.
Thats what I don't understand, an 6570 has the same amount of Transistors and Cores than a 6670, but the 6670 just have a higher clock, so if I overclock the 6570 to match the 6670's Clocks, I should get the same performance, no? Isn't that logic?
Correct me if I'm wrong...
Also it is harder to overclock the 6570 to 800 because Catalyst Control Center only goes up to 700, which the GPU will easily reach, then you have to use other programs to overclock further. At least this is the case with the 5570 if I am wrong then someone please correct me. But I would just pay the extra money to get the 6570 if you can't get the 6590. What is you price limit? $100?