When I crop, how much?

Just wondering if someone has a very simplified way of explaining how I can figure just how much I can crop from a photo (Digital Rebel) before it will start to look too bad for printing?

Also wondering about going from 180dpi to 300dpi in the photo without having the program resize/resample since I don't want to alter the image.. right?

I think I figured it out. I can uncheck 'resample image' when in the Imagesize Window. This will change the picture from a 17"x1"@180dpi to a 10"x6.8"@300dpi. Now I have a better idea of just what I can crop to have it stay at 300dpi and still look good. Am I Rite?

The smaller the picture the closer people will be so the higher the DPI. For a 4x6, 300 DPI. For 5x7, 240-300 DPI. 8x10, 240 DPI. 11x17 or bigger, 180-240 DPI.

That's a general rule of thumb, your mileage may vary, etc, etc.