Hello fingerz and welcome to the forums.
I suppose we should thank milou as well for pointing you in our direction
I can't help you with a direct comparison of the 18-55 against the 17-40 but I can recommend the latter very highly, I use one as my main lens for the majority of my photography. I am however a little confused as to why when you already have the 18-55 and 50mm lenses you would want to add another lens that covers approximately the same range?
If its for quality then the 17-40 L is about the same as the 50mm prime that you already have but allot of that being apparent will depend on your photography skill and being able to get the best from your equipment.
As a relative beginner (from your opening post) I would guess that the two lenses you already have would be good enough to tide you over till you have some more experience with the 300D and only after finding the limitations would spending money on a better lens to replace your current ones be a sensible move.
All the above assumes a lot on my part, maybe you would like to post a few pictures in the Photo sharing section of the forums so that we can see how best to help you?