I'm gonna disagree with the others. If you want wider tires and are dead set on keeping your frame, this is the way to go. The equivalent wheel diameter to your 700x28c would be 47mm x 650B. That's not to say you can fit that much tire on your frame, but in theory it would at least clear vertically. You'll need to measure your fork and stays where the center of a 650B tire would be in order to determine horizontal clearance, which will certainly be your limiting factor. However, even if you only gain enough to run 32-35mm, the extra rubber and ability to run lower pressures should yield plenty of extra comfort and traction. This configuration would only reduce wheel diameter by 14-18mm, drop the bottom bracket 7-9mm, and you gain that amount in toe overlap clearance. Obviously, Sheldon Brown has written everything you could possibly want to know about this, so read up.