I had the same problem with which lens to upgrade to for my 350D. Spent hours on the net. Go to
http://www.photozone.de website and they have a large number of reviews for canon, nikon, pentax and sony lenses and are easy to understand.
I would of loved the 24-105 f4 L but out of my budget so I bought the Canon EF 24-85mm f/3.5-4.5 USM which on the 350d is 39-136mm. A bit of a compromise on field of view but very good image quality.
Out of all the reviews I read for 18mm to XXXmm lenses, you cannot have sharp pictures throughout all the zoom ranges. Hope this helps.
Regards
Kon