It really depends on what you want to do. And there are so many variables to include that comparing raw numbers simply does not make sense: efficiency of a graphics card depends on many more factors than just only on clock rates and amount of on-card RAM. For example, chip design plays a major role, and it cannot be quantified (think what difference is between your i7 3770k and, let say, core 2 duo with the same clock rate, amount of memory and cache).
You say that the integrated card is pretty good. Does it then fulfill your needs? Why buying an additional piece of hardware if the current one might be enough for you?
If you want to play games—just look at the benchmarks of frame rate in these games. Long time ago Quake 1-3 were most often used for such benchmarks, recently I heard that games like Crysis are used. Just googling around might give you enough information.
If you want to use GPU for calculations, then that's totally different matter! Bitcoin mining works three times as good on AMD cards than on nVidia cards because of chip design. But nVidia triumphs AMD on more complex calculations. Also while nVidia shines when running CUDA code, it's not as good at OpenCL code, whereas AMD cannot run CUDA, and their OpenCL implementation is the best one available.
Also look at power consumption. Your current integrated graphics will be much cheaper in energy usage than a hefty GPU designed for heavy gaming or computing.
As a summary: each use case is different. Measure for yourself or look for benchmarks that test your specific use case. There is no other way.