You could compromise by going with the
Sigma 17-70 2.8-4.5 EX lens. I like this lens a lot and it's designed for the 1.6 crop sensors (Canon Rebel, XT, XTi, 20D, 30D, 40D, etc). At 17 mm, it is really good in low-light at 2.8, by far the cheapest you'll get at that focal length.
The Canon 24-105 f4.0 L that Mike mentioned, is nice too, but 3 times as much. I also own the
Sigma 10-20, which is about 100 more, and I love it. I rarely use anything else chasing, unless I'm not very close to the storm. The 10-20 is awesome with the HSM motor, (vs the USM with the 17-70), it's dead on and quick with the autofocus, silent, and takes pictures good enough for me to like. I owned the 17-40L and wasn't happy with it, (bad copy) and returned it to BH with no problem. So for 100 dollars more, and some discount codes I found, I bought two lenses I love for the price of an "L" series lens, which IMO, isn't worthy of the red stripe on it. Not to say that Mike doesn't have a kick ass copy, but I hear the newer ones being kicked out, are faulty. But if you buy from BH, you can return it within 14 days with no problem. They are pretty reliable.