The old one is showing signs of wear, so out with the old, in with the new.
Good advice BB,Mr pimpernel,I dont know your level of knowledge of photography, so forgive me if I'm suggesting stuff you already know !
First, get yourself a tripod. Even a cheap used one off ebay for a tenner (like wot mine is) is perfect.
Next, shoot at something like f8 or smaller, this will give a sharper shot with more in focus.
Next, the reason for the tripod. Take 2, or even 3 shots, at different exposures, without moving the camera, AT ALL ! You can use bracketing on your 500D for this, or simply adjust it manually usinging the thumb wheel. You can then combine these shots in an editing program to give a perfectly exposed shot.
Hope this is some help as you may find you dont really need a better lens ;-)