I will add that
www.slrgear.com and
www.photozone.de also give great advice on various lenses.
Most lenses will indeed be better than the kit lens of your Canon. The kit is not bad, but as with everything you get what you pay for, and by paying more you can get a higher quality lens. That being said, at f8 most lenses are excellent.
But I remember some time back that a Wide angle zoom lens offers a better picture.
There is no direct relation between the focal length of a lens and its quality. A lot goes into the design of a lens, and a "better" lens is usually not that easy to define. Contrast, sharpness, saturation, distorsion, aberrations all play a role in the final image quality.
As Gail recommended, visiting the user and pro review sites is probably the best way to find some information on specific lenses.