Regarding DPI:
640x1016 pixels, 68 DPI (Dots Per Inch), 9.412x14.941 inches ...
640x1016 pixels, 500 DPI (Dots Per Inch), 1.28x2.032 inches
As you can see, DPI has no effect on either quality, nor filesize (nor anything else, really) while the image is on your computer. DPI only takes effect when you go to print the image. Since you can adjust this after the image has been saved (in Xnview, hit shift+S, then look under "print size" and you'll see "units" and a dropdown box for pixels per inch or pixels per centimeter. Leave the dropdown box at pixels/inch, then whatever number you put in the blank for "units" is what the new DPI will be), it makes no real difference what DPI you choose when scanning. Typical DPI for images which are intended for computer use is 72, typical DPI for printing is 360, I print out photos from my digital camera at 120 DPI and they come out very nice (I think 360 DPI is MASSIVE overkill).
For instance, a typical 8.5x11inch paper scanned at 100% and 100 DPI will result in a 850x1100pixel image. The same document scanned at 50% and 200 DPI will result in an identicla 850x1100pixel image. There will be no difference between the 2 images while they're on your computer, but if you print them without first changing the DPI, the second one will print out at only 4.25x5.5 inches instead of 8.5x11. It should be noted here that when you choose a size in the print dialog (in most cases, you can choose "actual size", "best fit" (which makes the image fit the page as well as possible without changing the height/width ratio), "fit to page" (which stretches the image to fit the page, ignoring the height/width ratio), or various percentage), this will override the DPI of the image. So, if you take that second image, and tell it to print at 200%, it will print a 8.5x11inch image, just like the first one would at 100%.
NOTE: There is a bug/flaw in Xnview's resizing. The resizing filter should not be applied when the dimensions of the image don't change (ie, when you're just changing the DPI). However, Xnview does apply whatever resizing filter you have selected when you just change the DPI. This usually results in a somewhat blurred image. To avoid this, choose "nearest neighbor" from the "resample" dropdown menu.
ALSO NOTE: In most programs, the screen size will remain the same when you change the DPI, but the print size/actual size will change accordingly. In Xnview however, the print size remains the same and the screen size changes with the DPI. (This may be the cause of the bug/flaw mentioned above.) To avoid winding up with a massive image, you should take note of one of the current dimensions of the image, set the DPI, then go back and set that dimension back to what it was.
I realuize some of that sounds complicated, but when you actually do it, it isn't. It's just one of those things that'seasy to do, but hard to explain.
