Attempts to print at that resolution will probably generate either a black hole or a Strangeling Particle that will quickly consume the entire Universe. However, the print might look OK. Ignoring the Common Wisdom, I would tend to upres such an image to either 240 or 360 in Photoshop, and possibly even try to sharpen it a little bit using Smart Sharpen with the Highlight and Shadow sliders set up to about 14 or above, or to the smallest numbers where Shadows just barely stops looking different, and where Highlights doesn't generate halos at contrasty transitions. Sharp and relatively low-noise 6mp files can make surprisingly big prints. Also helps to have an interesting subject, which can take a lot more blow-up than a boring subject.
This is the kind of thing where an actual Test is worth a thousand expert opinions. Crop out a 3 x 13" section of the image and, one at a time, apply and print every type of the upsizing technique you have ever heard of to that sample. Emphasis on the "print" part. Just keep chomping away at the same piece of say 13x19 sheet, each time slicing off the printed part. Make lots of notes. Tape the prints up on the wall and view at various distances, including "normal" distance. Expect the best method to be not what you expected. Works for me.
