StewartR just yesterday pointed me to a great statement on this question from another thread:
Jon wrote:
If you're wondering whether you'll benefit from getting the newer camera, you probably won't.
The time to change cameras is when you know exactly why you want to ugpgrade to the newer one.
Yes, the 30D (or a used 20D, by the way) will perform nicer (faster, more convenient) and offer you slightly better image quality than your 300D. So will a 350D or 400D (faster).
However, you have to completely compare the 2 options:
1) New 30D + Tamron 18-200 ... more convenient, faster, marginally better image quality
2) old 300D + new lens (e.g. 70-200/2.8 L or 24-105/4 IS L or some good primes) ... slightly better handling (faster AF, brighter viewfinder), and most likely dramatically improved image quality
So my vote would go to better glass, wait maybe another year or two, and upgrade your camera when you feel you have reached its limits.
Best regards,
Andy