First of all, get yourself a EF-S15-85mmISUSM to experience most used focal length and quality of pictures this lens produce. Then, you will have a better idea of what focal length is your most used focal length.
Once you have determined your most used focal length then you can start to search for prime lenses with large aperture size. You will soon find out the attractiveness of prime lenses.
Once you have experienced the picture quality of short focal length primes then you can ask yourself "do i need to buy 17-50mmF2.8IS"? I am sure you will save yourself lots of money and enjoy more of your photos.