Sukesh, buddy, I have posted several times that a true 350W PSU will give 350W of power output irrespective of its efficiency. Efficiecy is completely different thing...It is the amount of power that will be fetched from your home circuit by the PSU to provide 350W of power output.
Suppose X Watt is the power required by the PSU to provide 350W of power. Then
X* 80% = 350
-> X= 350X100/80 = 437.5W
So the PSU will take 437.5W of current from the circuit to provide 350W of output. Make sure you understand it and don't get confused. Efficiency helps you to reduce the power consumption as more tha efficiency, less the power required by the PSU to provide its rated wattage.
For HD6850, minimum of 450W PSU is required and FSP SAGA II 500W @ 2K is the best choice. Another thing, currently all the Phenom II X4 955 comes with C3 revision and have 95W of TDP, not 125W.