550D will look and behave a lot like your 350d (they are both xx0D for a reason). In other terms both have the smallest viewfinder, about 3fps, and tiny body with rather awkward controls. Unless you are happy with that xxD can offer some improvements, like bigger VF, 6fps, more comfortable body, stronger built, etc. 40D might be enough, I really couldn't find anything much better 50D. The important question is what are your lenses, and can they cut the mustard?