I'm planning on moving to either 25mm front and 28mm back or 28mm all 'round. And I know that I can just barely squeeze 32mm commuter slicks on my existing rims/brakes/frame. The reason I wish to move to wider rims is that they give an increase in air volume for any given tire as compared with narrow rims. And, since I'm moving to larger tires for speed on the chip-seal and comfort on partially patched potholes, more air in a given width is a winner.