I have a D5100 too. I upgraded to 50 MM 1.8 Nikkor. and recently to yongnuo 568 EX speed light.
Speed light has given a whole new dimension to the kit lens 18-55 VR.
Next upgrade i intend to do is 24-70 SIGMA 2.8 or 17-70 SIGMA 2.8/4. Also will get the wireless trigger and another basic speed light which would work as a slave from my camera.
Sharing this info coz i expect that it will trigger some discussion and we might get to learn something new from the experts :).