Steve you have listed cameras from 3 different generations and are trying to compare them. Just to clarify
The 300D that you are currently using is from the same range as the 10D that you mentioned. The 10D was the 300d’s big brother and had several improvements, main ones being, all metal body, larger sensor, bigger buffer, higher frame rate and less noise at higher iso’s.
The 300D was replaced by the 350D as the 10D was replaced by the 20D. Again a similar upgrade pattern is visible, larger sensor, bigger buffer, faster frame rates and better noise. The 350D is still a plastic bodied camera and its size was smaller than the 300D it replaced. The improvement in picture quality could not be denied but some people find the 350D a little too small for the size of their hands. The 20D was also slightly smaller than the 10D but is regarded as a more useable size. Again similar improvements where made to the 20D over the 10D.
The 20D has now been joined by the very recently released 30D and is currently being phased out. Again the improvements/changed between the specs of the 20D/30D are well documented on this and many other websites.
That basically leaves you with a choice, if buying new, of a 350D, a 20D or a 30D but as you are looking to the summer then it all likelihood you will be down to two with 20D being all but history.
Does the 30D do anything that you feel you need and the 350D can’t?
If not then subject to the 350D feeling comfortable for you, (my best advice is to try before you buy, handle both to see how they feel), then any extra money you might save by buying the cheaper model could be put towards better/more glass which is where the real flexibility and quality is more likely to come from. As you will have seen Camera bodies are regularly updated and quickly become last years’ model, lenses on the other hand, if bought carefully will last you a lifetime.