You can't really look at it like that. There are a million other factors to consider (cores/ROPs, memory bandwidth, fill rates etc) and it's just impossible to look at those numbers and be able to say 'that translates to a 15% framerate gain'. To really understand how this stuff compares, you need to look at benchmarks. There's often a lack of consistency there - depending on the game, resolution and other settings, cards do better relative to the competition in some situations and worse in others.
You need to look at a whole load of benchmarks and get an idea of the average framerate difference across the board. You personally don't need to of course, because you've got us nerds already having done that :-) I'd estimate the GTX660 Ti would deliver around 33% more performance than your 6950. Sounds good (and it kind of is) but it's only the difference between 30fps and 40fps. Up to you if you think it's worth the cost. A GTX670 would be a better upgrade - I'd estimate over a 50% gain on what you have.
EDIT: What do you know - 1058 / 800 = 1.32 - a 32% gain :-) Maybe you can just look at clock freqs! Kidding though - don't do that.