If your sigma lens fits your canon body, then it will work with other canon EOS bodies, so yes it will work with the 350D. It won't work with Nikon or Fuji bodies. However, I'm sure Sigma makes versions of the exact same lens that fit bodies from other manufacturers, so it would likely be possible to sell your current lens and get one for your new camera for not too much money. That said, both the 350D and the D50 are great cameras (I'm not too familiar with the E-300), so you should probably be happy keeping with Canon instead of dealing with getting new lenses for Nikon or Fuji.