Thank you very much for your answers so far, I really appreciate that! Looks like it's an almost unanimous vote for the 17-40. I sure wouldn't mind trying the 16-35 but as stated above this is way out of my budget right now. So I guess the 17-40 it shall be then.
As for the Tamron XR Di 28-75 f/2.8: I really like it as mine delivers very sharp photos even wide open, and that not only on the EOS 300D with its crop factor but also when I shoot full frame (mostly slides) with my EOS 33. The autofocus might be little slower than Canon's USM
but for me this is no real drawback. The built-quality of the lens is good, and as far as I can tell you get as close to Canon L-glass qualitywise as you can in this lens-range, but for 1/3 of the price. In a nutsehll: I wouldn't want to miss mine anymore.
HOWEVER: Beware that apparently there are some bad copies floating around which have a sharpness-problem. So either take your camera to your local photostore and try out the exact copy you want to buy. Or if you order it online make sure that you can return it in case you get one of these bad copies.
Finally, to give yon an impression of the lens's performance check out some of my photos taken with it. I tried to include different light-conditions and focal lengthes.
All photos taken with Canon EOS 300D and Tamron XR Di 28-75 f/2.8
Best regards and thanks again for all your hints and tips!
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