Question about enlarging digital photographs.

[Silent3]

13. You should be fine then.

That will yield an end result with 3456/40 -> 86.4 ppi. While that's not super sharp, it's not bad looking either. With the typical smoothing that's done when scaling, and plenty of target pixels to render the original pixels, you'll get decent results.

It's also unlikely you need to mess around with Photoshop or anything else. Whether you're using your own printer, or having a commercial service create the print, just specify the size you want the end result to be and the printing software will take care of the rest.

Edit: Seeing what Stevenmarc said about Perfect Resize: Yes, you certainly can use software like that to do the scaling before handing the job off to whatever printing process you're going to use, if you want/need to squeeze out every last bit of possible image quality you can get, but you'll probably be pretty happy even with most of the standard default processes.

Please realize that (regardless of the impression TV cop shows might give you) the original resolution of a picture puts a pretty big limit on the real information available in that image. Typical scaling processes (and even this Perfect Resize) can't restore very much image information that wasn't directly visible in the first place. Mostly all you can do is generate various versions of eye-pleasing fakery, either in the direction of gently smoothing out pixelation and/or generating artificial but convincing simulated detail.