Dots per inch question

I once knew the answer to this, but I forgot. I use a Nikon CoolPix, but I
also have slides scanned to Kodak's photo CD. The show up in my image
editing app as 72dpi, and we just got a printer that claims to print up to
4800 x 2400 dpi. I can set the resolution in my image editing program, so
I've just re-set the 72x72 as 300x300 and printed a scanned Photo CD image
at 13 inches x 19 inches, and it's a stunner. (Uh, good stun, not bad
stun.)

The files in the Photo CD are .pcd, so I assume there's 4800 x 2400 dpi in
there somewhere, right? I can set my image editor to open at that
resolution and get it? My wife has a digital Rebel, and we'd like to get
maximum results from that camera, as well.

But what about the Nikon CoolPix images? Do I need one of those fractal
interpolators or whatever they're called?

(I realize whether anyone can tell the difference between 300 x300 and 4800
x 2400 is a different issue. I'm just trying to get a handle on how to
handle dots per inch. As a film shooter, this is not an issue I'm familiar
with.)
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