I still use my 300D (had it for close to 6 years now) and I can still get some amazing shots with it. It really depends on the types of photos you take. The one thing I don't like about the 300D is that its so slow so if you need something fast then definitely get a faster camera even if its just the 350D or the 20D, I think its a big improvement on the 300D. By 'slow' I mean you can only take 4 shots before the buffer fills up and if you're shooting in raw it takes a while for it to empty it to the CF card. I was taking some lightning shots a few months back and I missed so many good shots because the 300D took its time to save to the CF card. But other than that, it still gets the job done and I still use it a lot.
If you are to upgrade you can't really go wrong with a Canon or Nikon. They both make brilliant cameras. If you're mainly a landscape guy and you're still happy with your 300D then I don't see why you can't stick with it. I still love mine and it was my first DSLR so I'm never getting rid of it.