I think how powerful a graphics card you need depends on your expected usage of UE4. In my case, I am a beginner still learning so I can get by with my GTX 650 Ti Boost no problem. Even in higher end stuff - like tech demos - I still have no problems viewing those through the editor with the graphics card I have. So going with the recommended GPU from the UE4 page would be fine for most people, unless you are planning to do something really big that requires a lot more GPU power for testing and developing in the editor.