Your CPU is not good enough for a HD 7850. It will bottleneck it. Believe me, I know what I'm talking about. Even the FX-6100 is to weak to handle that particular video card. Don't get me wrong, it will work, but you will only be able to sqeeze approx. 70-75% of the performance of a HD 7850. Therefore, I would stick with an 650 Ti, because at least you won't pay for perfornance you will never be able to benefit from.
UNLESS you upgrade to a FX-8350.
FYI: If no bottleneck is involved, the difference between HD 7850 & 650 Ti is on average 13 frames per second at 1080p MSAA x4.