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This is a question which does not have a definite answer. The cards perform differently, depending on which game they are playing, and how the game's programmers used the game engine. The only true test you can use is how one card compares to another in a specific game, with specific settings.
That said, you can refer to benchmarks to get a general idea of how cards fare against each other. I don't look at benchmark sites very often, so I might not be able to give you the best sites. The sites I used to look at were Anandtech, Tom's Hardware, and Passmark. Guru3d is also a good resource, but not so much for benchmarking. Start there, or look for your own favorite benchmarking site.
Among current generation cards, from highest to lowest performance, it's something like:
GeForce GTX Titan
(big gap here)
Radeon HD 7970 GHz Edition
GeForce GTX 680 ~ Radeon HD 7970
GeForce GTX 670
Radeon HD 7950
GeForce GTX 660 Ti
Radeon HD 7870
GeForce GTX 660
Radeon HD 7850
GeForce GTX 650 Ti Boost
Radeon HD 7790
GeForce GTX 650 Ti
Radeon HD 7770
GeForce GTX 650 ~ Radeon HD 7750 (with GDDR5, not DDR3)
GeForce GT 640
click on any card and a list of cards will appear on the right side of your screen sorted by performance and class level
select any card from that list and you will get a plethora of data.
I typically look at performance on games I am familiar with like GW2
I just purchased an Asus G75vx with a GTX670mx in it. I think I will get plenty of time off of that card and it's all wrapped up in the quietest laptop I have ever seen.