The 7770 is much more of an entry level card, however, is ideal for upgrading computers that may not have the sort of power supply and cooling required to handle "big gun" GPUs. The 6950, is a much more powerful card, but at the cost of much higher power consumption. The 7850 is more or less the "replacement" for the 6950 and uses less power to effectively deliver the same performance and would be the smarter choice unless the price gap was massive where you are.
We will be able to provide much better advise, if you provide more information about the system that is being upgraded, what sort of budget you have, and where you can buy from.
Click on "rigbuilder" in the upper right. Input as much information as you have.
Edited by mdocod - 8/12/12 at 1:41am