I love to have lenses, and love to have a lot of them, but I like your idea, and I think you'll do great with that setup. The 17-55 is a great lens, but it's and EFS. With the 17-40 you can move to a FF, and keep shooting.
Don't buy a lens for a body you might have someday, buy one for the one you have now. Any of the higher quality EF-S lenses, like the 17-55, 15-85, 10-22, and 60 macro; all hold their value well. If you buy used, and it sounds like that's the budget range you're working with, you'll probably get back what you paid for them anyhow.