Think of it this way.

a image that is 10 pixels per inch is printed and it comes out 10" x 10" on paper.  The size of the image would be 100 pixels by 100 pixels.

a image that is 100 pixels per inch is printed and it comes out at 10" X 10" on paper.  The dimensions of the image would be 1000 pixels by 1000 pixels.

if you were to not resample the image and change the dpi in the first image to 20 pixels per inch then the printed image would be 5" x 5" but the pixel dimension of the image would be 100 x 100 pixels.  Nothing changed except the parameters we told it to print.  The same with the second image, if we changed it to 200 pixels per inch then it would print at 5" x 5" and still be 1000 pixels by 1000 pixels.  Nothing in the image actually changes.

If we set it for resample and we change the DPI then the inches stay the same but the number of pixels change.  For the most part ignore the DPI and set the pixels you want for the output size, then if you are going to print and need to change it, do that in your photoshop.  So if you feel that you need 300 dpi to print or 250 or 200 then calculate what ou need.  THere are also some outside of photoshop enlargers that do a very good job on many types of images to keep out the blotchies.  As an example if you wanted toi print a 10 X 10" image and you think that 200 dpi is good enough for that print, then you would need an image that is 2000 x 2000 pixels to start with.

DPI is only relavant when printing or if your software uses it to change the actual pixel size of the image.

In Carrara if you have Keep Proportions checked and you change the DPI the inches stay the same but the actual number of pixels change.  If you have it unchecked and change the DPI the actual pixels stay the same but the inches change.

The Keep Proportions in this case for the DPI is like the resample for photoshop below.

See where in the two images the little link chains are tying together width and heigth, but not in this case dpi?  Now if you change the DPI right now it it will change the number of pixels but not the size in inches.  If you take off resample, the little link then ties into the dpi as well.  Now if you change the DPI the pixels don't change but the size of the image does.