IF you can, hold off on mods and invest in some WORTHY lenses...
I recently got the 24-70mm f/2.8 L and I love it to bits. The build quality, the fast focusing, the sharpness of pictures... one of the best lenses to have with a decent focal range.
Here's a site that I always refer to when looking up lenses. The author writes decent reviews and also posts pictures he took with those lenses.
http://www.the-digital-picture.com/Canon-Lenses/. Go towards the bottom for specific reviews.
I must say... once you go L, you probably dont want anything else. My next purchase will be the 70-200mm f/2.8 IS L and then the 10-22mm f/3.5-4.5... from there i'll start moving away from zoom lenses for specific needs (fisheye, the monster 300mm+ lenses for wildlife, etc.). I only got into photography a year ago. my suggestion is to read a LOT and make sure you buy lenses that suit your needs (like everyone else said).