I´ll add my 2 c. as a 10D shooter for over 2 years with a bunch of lenses( more than I thought Ill need). Keep in mindthat Ill give advise without knowing what kind of photography youll make.
- 24-70. I own and LOVVVE this lense. Its big, heavy, expensive and not very wide in an APS sized sensor...but to my surprise it´s the lense that spends the most time on my camera. It might not help a lot but the majority of the "great" photographers of the 20th century never went any wider than 35 or 40 and still manage to produce stunning images...I believe that having just one lense(even with the limitations in range) will help to develop an "eye", and give power to your compositional skills. I´ve found that owning so many lenses can be very distracting and gets in the way of getting the shot.I sustain the theory that shooting with limitations and difficuties(for a while) will make you a better photographer..you might not agree. The images out of this lense are beauifull, its fast and the bokeh is very pleasing.(That´s why I voted for it on your poll though I admit that the 17-85 is probably a more clever decission.)
-50 1,8. This lense is very cheap and makes a perfect complement to whatever you get.Whatever you decide, get one. Its sharp, small and a pleasure to use.Perfect for when you want easy,light,fun and laid back shooting.also great for low light situations.
17-40 f 4- When I bought this lense I was obsessed with the wide angle coverage. I,ve found to my surprise that I use it less and less.Dont get me wrong, its a fantastic lense, but I would´nt recomend it as the only lense in your arsenal. Its slow and the range is to short(for my taste).
17-85. Though I don´t own this lense,logic says its the right decision.Youll manage to cover the majority of the situatios with it, small, not to heavy and I believe the IS is tops. It seems to be the perfect walk-around/holiday lense and the one with which youll probably be less limited.If you decide to get this I still recomend you to get the 50 1,8 or even 1,4.About it being an EF-S lense, if you ever get a full frame or anything else I dont think youll have any dificulty selling this lense with your 20D body and I think this EF-S´s are going to become very popular and will be among us for quite a long while.
I used to "suffer" about which lenses I would be buying and in which order, and I´ve burnt my eye lashes reading EVERY review available on the net.Now that I own everything I wanted I find myself walking the opposite road, trying to simplify and allways wanting to carry the minimum stuff.Less gear means more energy spent in shooting and getting the images...I suggest you visit this site as an example of what can be achieved with just one camera and one lense(a 50mm).You might like his stile or not, but there will be no doubt about the comitment.Heninger photographic
Maybe we are spending to much time on this forums...
I hope this will be of some help and sorry for the long post..
good luck with your choice and most important: ENJOY!!