Assuming you aren't planning on upgrading to a full-frame sensor in the near future, I'd say keep the 70-200 f/4 and get the 17-55mm f/2.8 IS. Everyone on the forums here knows how fond I am of that lens. I think it's the most useful general purpose lens on a crop sensor body. It isn't perfect--but it seems much more time on my main camera than any other lens I own. By getting the 17-55mm f/2.8 IS (and selling the 18-135), the only thing your losing is a bit of focal length range between 55-70mm, and IS beyond 55mm. In my opinion, the extra stop on the lower end is worth the trade off.