6850 is out of range
Ok, if 6850 is out of range, I would go wit the 6790.
another would be gtx 550ti which has physx but ati has no physx so am i doing right thing of buying ati ??
There are very few games that take advantage of PhysX, and given the performance level you are looking at, you would likely turn it off anyway to get better detail/resolution. You don't need to worry about PhysX.
You won't even notice that you don't have it.