You just cannot go wrong with the 17-40mm. While slower and a little less range than the 17-55, it will make the jump with you when/if you go to full frame and you will not be disappointed. That said, what about looking at some fast primes? For about the cost of the 17-40, you could pick up a 35mm f/2 and a 50mm f/1.4. I would think about what you think is technically limiting your creativity at this point and buy to compensate. If your 18-55 lives on the camera but you just aren't happy with the results, buy a mid-range zoom. If none of your lenses function well enough in low light, fast primes.