I can not buy any other brand. These are the only place I buy there. They are the cheapest. "640, it will be a bottleneck for a 6870." I will pay attention to it.
Maybe I'll buy POWERCOLOR HD6850 GDDR5 1GB 256Bit AMD Radeon - İt's only 119 $. How can this be an option?
If you overclock, then a 7770 might be a better option. Good models can often overclock significantly further than a 6870 (granted that's partially because 6870s tend to not overclock exceptionally well) and at 1680x1050, the memory bandwidth loss from going down to a 128 bit GDDR5 memory connection shouldn't hurt much at this performance level.