Also how long the image will appear to your audience should dictate how much should be on it. They can't listen to you and read your visuals at the same time. Beyond Text Slides and overheads do not always have to be text, charts, and diagrams. Originally slides where photos. They add excitement, color and usually can portray an object or location better than text.
Overheads can also be photos.
The quality varies from high to low depending on the method used. E-6 8x10 transparencies have the quality of 35mm slides and a steep price tag too. Color copier overheads are cheaper, but the quality is not as good. Scanning an image and printing it to a high quality color printer loaded with transparencies is another method. This takes more work, and file sizes are usually large.