Ok, well DPI and PPI are related in the fact that when you are working in a imaging program it is considered PPI, when you are printing it is DPI. If you plan on printing these images, 300 should be the min. resolution, when I work with print for work, it's 600 DPI. Now, when you are taking images with your camera, and it has a setting for DPI/PPI, exporting the image might be a problem (im not sure), but, if you import to an imaging program at 72 PPI, and try to make it 300 for print, it's going to be extremly distorted. So if you have a setting on your camera for resolution put it to at least 300 (if you plan to print), you wont see the difference on the screen (because screen resoultion is 72 PPI), but it will ensure the picture to be a quality one.
The website markc posted is wrong, don't listen to it. I will prove it now.
Goto google, click on images. (the reason for this is because most of the images found on the website are web images (72 dpi)
Search for anything get the full image, and copy the image to the clipboard.
Next open up Photoshop, hit apple+N, (cntl+N windows)
Now since the image is in the clipboard the image size is already set for photoshop, hit ok.
When the .psd is open paste the image apple+V (cntl+V windows)
next goto Image>Image size.
Change the DPI setting to 10.
Look at the image...
Now go back to Image>Image size, and make it 300.
look at the image at 100% and tell me that doesn't look like sh!t.
Then, try to print it, lol, looks like colored mud all over the page.
PPI/DPI matters A LOT.