i doubt its a power issue for several reasons
1. the gtx560 and amd 6950 are in the same ballpark in terms of power needs
2. you powered the GTX295 with no issue and that card needed far more power
3. your system with the AMD6950 won't break 350w gaming so your TX850w has plenty of power to spare
did you delete the nvidia drivers prior to installing the AMD drivers?
No i did not...i do doubt too... But the light is not convincing me! Can it be the psu being an old model and not fulfilling the requirement? 12v w 30a