Aaron B , Sep 17, 2005; 05:41 p.m.
What matters is overall resolution of the images and the resolution of the displays you will be using. To be a stickler, you would normally want a final image resolution that is as close as possible to the resolution of the projectors you will be using without going over. If your pictures have more resolution than the projector, the slide-show software will be forced to scale them down. Although the routines they use to scale could be of any quality, I've never really seen that it was enough of a problem to worry about.
Still, if you're not planning on saving the images or using them for anything else, to save time, you're better off just scanning to match your projector resolution.
For example, if you are scanning a landscape format 4x6 picture, for display on a projector with a resolution of 1024x768, you want 1024 horizontal pixels. So, you would scan the picture at about 175 DPI (or higher and then crop & scale the image back down in Photoshop). If you're scanning a 4x6 in portrait orientation, you would scan it at around 130 dpi (or higher and then crop & scale in Photoshop).