You also dont really need to print at 300 dots per inch, you can resize the photograph to around 225 or 250 dpi which when printed would look almost identical to the 300 dpi image.

You have an approximate 3mpx image and your question is comparing to 35mm. I have always read that 6mpx images are considered to have about the same resolution/quality of a 35mm film. With technology being what it is however you can upsize the image with little to no loss in quality.

Edited to add:
Also mentioned above is the "viewing distance", large photographs are meant to be viewed from specific distances. If you hold an 12x18 image 3 inches from your face it will not look the same as hanging it on the wall and viewing from 6 feet. Too many people will inspect an inlargement inches from their face, however unless it is being judged that really is too high an expectation.