a simple google will do for that, but there is one thing that isn't mentioned or is overlooked by alot of people;
HD 6950=GTX560Ti performance wise (
chart)
GTX560Ti ---> Physx (capable fo calculating it)
HD 6950 ---> they are yet to incorporate this feature into their chips
HD6950 price<GTX560Ti
the radeon 6xxx run far,far ,far cooler than any Nvidia released to date. The only dif in the nvida camp is they run cooler than their predecessors.
* accoding to tom's chart - if you're getting a GPU upgrade than you should look at a card at least 3 tiers higher or else you get near similar performance making the purchase futile - i can see you already have a 9800GTX+...so you seem to be good