Upload & Sell: On
The 70-200L in any version is killer.
The 70-300 IS is even cheaper and is a nice lens with a bit more reach
If you don't need the reach, perhaps the 100 f2.8 non-L macro, it's super sharp, plus you get macro which can open up a whole new world.
Don't forget extension tubes and/or tele converters/extenders as an option for you current lenses.
Like you I shoot crop bodies, my regular kit is a 40D, 10-22, 50, 100mac and 70-200, with a 1.4x. Your 28-75 is a nice lens, I'd probably get the 70-200 for the constant apreture; or the 100mac as it's one of my most used lenses. Look in the Buy&Sell forum for used gear, prices are quite good...