Ron's got the way of it. However, if you are going to print them at all, use 400 or better dpi for magazine work, 200 dpi for newsprint (which is the norm ... but a total waste. I've never seen a web press that could get anywhere close to that detail. <..
..> ) or scanner max for serious photo printing. Then when you pull it into your GIMP or Cinepaint editor ... you've got some detail to work with. You can always shrink them down from there. (Depending on the shot and the starting size, it's best to shrink in stages, and adjust sharpening and contrast after each stage.)
A caveat, though. It's incredibly easy to over-run your available ram when working with large images. Best to trade off detail for ability to actually work with the image. I use a full 2 gigs of ram on my image editing machine.