UPDATING FROM CANON 300D

My current camera kit consists of:
Canon DSLR EOS 300D (REBEL), EFS 18-55mm F3.5-5.6 (300D Kit Lens), EF 100 mm f2.8 Macro USM, EF 75-300mm F4-5.6 IS

I have been learning photography for 2 years now, When I began, I purchased a camera and lenses by what I was told, not by what I knew. Now, I am looking for an excellent mid-range lens (CANON only) to compliment my lenses, as I do not want to use my EFS lens anymore. Something that is a good walkaround lens, that is able to replace the range between 17-100mm. (I am pleased with my Macro and telephoto, for now).
If I purchase an expensive mid-range lens, will I still get the lens quality results using my Canon 300D? Or should I get a new camera and mid-range priced lens? My budget is about $1,500 - I can go a bit over. Any suggestions would be helpful. (I do prefer suggestions for Canon lenses only).

Go and get a 5D