Sounds to me like no need to switch cameras. Read up on some exposure basics and/or take a beginner DSLR class via a local camera store or community college. I have a friend with the same camera that you use and she gets lots of great photos with it.
One other thought. Take your camera to the local camera store and take some test shots both indoors with flash and outside without flash, then grab any of the rebels you list in your original post and take the same test shots using the same settings. Compare the results. If they both have the same problems, revert back to the suggestions of reading up on exporsure and flash usage. If the Canon shots are good and your Nikon shots are bad, there might be something wrong with your camera. However if you are shooting in auto, you will need to compare the exif data for the shots to see what settings each camera is selecting when in Auto. The salesperson can likely help you with this.
Last edited by Flossmoor40D; 09-01-2009 at 01:34 PM.
Canon 40D - Canon EF 17-40mm f/4 L - Canon EF 24-70mm f/2.8 L- Canon EF 50mm f/1.8 II - Canon EF 70-200mm f/4 L - Canon 430 EX II Speedlight