If you shoot mostly wide and mid range and you want to eventually add a full frame DLSR I think that the
EF 24-105/4L IS USM might be the best choice. It's a great wide to mid range zoom on a full frame DSLR, though on a 50D its more of a semi-wide to short telephoto. Still a very useful lens though. If you want wide on the 50D I'd look at the
EF-S 10-22/3.5-4.5 USM. With that and the 24-105 you'd effectively cover everything from 10 to 105mm. The EF-S 10-22 won't fit on a full frame camera, but if you want to go really wide on a crop sensor body, you pretty much have to use a lens designed for crop sensors.
The
EF-S 17-55/2.8IS USM is a great lens and gives you good wide-to-normal coverage. It's fast and it has IS. The only downside is that if you add a full frame DSLR, you won't be able to use the EF-S 17-55 on it.
The
EF 70-200/2.8L IS USM may be Canon's best zoom. It's very sharp and the IS is effective. It can be combined with the 1.4x and 2x TC for even longer reach. However you said that you mostly shoot wide and mid range and the 70-200 is really a telephoto zoom, espcially on a crop sensor DSLR.
The
EF 24-70/2.8L USM is a nice lens with a wide aperture that can be useful for throwing backgrounds out of focus. The only real downside is that it lacks IS, so you can probably shoot handheld with the 24-105/4IS in lower light than you could with the unstabilized 24-70/2.8.