I've had both the 18-55 and 17-55, and "not a significant improvement" need to be clarified a bit. The plastic and cheap 18-55 is quite a bit better than it's price might suggest, and if you would shoot these lenses at f/5.6 or f/8, the results are not going to be very different. However the 17-55 is not only an excellent lens, it also allows you to shoot at f/2.8, and the look of a portrait taken at 55mm and f/2.8 is dramatically different than at 55mm and f/5.6.
If you stick with DX, I would not go with the 24-70. At some point I moved from the 17-55 to the 24-70 (when I was planning to move towards FX). Not only is the 24-70mm focal range quite impractical on DX, I did not find the results to be noticeably better than the 17-55.
Finally, what if you don't need/want f/2.8? Then check out the 18-105 which is a nice improvement over the 18-55 while also adding the range from 55-105mm.