Photo printing guide - What resolution to use per print size

I often find myself trying to figure out what resolution/file size I should use to print photos.  I came across this guide from the blog Urban75. I'm posting it to keep it as a reference for myself.  Maybe you'll find it helpful as well.

For printing on ink jet printers:

Professional/online printing
Some professional services specify lower resolutions for image sizes, so you could get away with blowing up your images.

Kodak, for example, suggests these resolution/file sizes:
For a 4" x 6" print, the image resolution should be 640 x 480 pixels minimum.
For a 5" x 7" print, the image resolution should be 1024 x 768 pixels minimum.
For an 8" x 10" print, the image resolution should be 1536 x 1024 pixels minimum.
For a 16" x 20" print, the image resolution should be 1600 x 1200 pixels minimum.
For a 20" x 30" print, the image resolution should be 1600 x 1200 pixels minimum.
For a Wallet-size print, the image resolution should be 320 x 240 pixels minimum.