what different hd6770 with hd6870?
The 6870 will give close to double the frame per seconds ( determines how smooth your gaming will be ) compared to the 6770 at less than double the cost. Therefore, it is the better buy.
can it support too wear of two gpu in one mobo of this two item..
Yes. Your motherboard can accommodate 2x6870. But the more important question is what is your power supply? The 2x6870 will require 4x6PIN PCIe connectors and recommended 600W capacity.
Why you want 2x6870 is still an open question. If you tell us the reason, we can give you even better option than these.
& what is you monitor resolution? Ex: 1920x1080?