I assume your question is if you take a photo from a D300 at 12MP, what you should do with regards to its size to get to the final printed output.
Since the topic is just about print size, this is what I will recommend.
When you approach your photo taking, frame properly to avoid unnecessary crop. Now it seems irrelevant to pixel count, but if you need to crop, that means your source image is smaller and you might just need to enlarge. Minimize this step will help get you the largest image you can get from the source. Remember that optical enlargement by your lens is the best quality you can get out of a scene versus any digital solution.
Using 300dpi as the final output, for a 4R (4" x 6") size, it would ideally be 1200 X 1800 pixels. If your image is somewhat smaller, then enlarge as accordingly by resampling with bicubic smoother algorithm. But before doing that, you will wanna apply noise reduction if the image is noisy to start with, that's why it's better that you approach the photo taking with as long ISO as possible to minimize on noise, because noise also get enlarged in the process. After the enlargement, if you find the image is soft, apply the unsharpen mask filter(yes it's "unsharpen" to make it more sharp) to sharpen your image to your preference.
On the other hand, if your image is larger than 1200x1800, then good scale down your image using the same technique but the algorithm is chosen as "bicubic sharper". It makes your image sharper in smaller size, and also less noisy in the process. You will wanna do this yourself to be more sure on the actually output, than to be done by the software in the print house because you cannot control their process easily.
Export your photograph as TIFF format because this is what I understand as the industrial standard, not JPEG, not PSD so forth for images. TIFF has all the necessary features in the file format to ensure consistent colour management transfer, layerings etc... Colour management is totally another topic to discuss on. To me, what the DPI is doesn't matter and I have came across print house don't bother too. You or them can do simple changes to this variable in the file or during printing in the software, what matters more is the pixel data. If you have insufficient data, you lost quality. If you have too much data, it can also be lost of quality when you scale down. The more quality lost when using bad scaling algorithm.
So to answer your question, resample your image in your own requirement before you send for printing.
Wow.. Learnt a lot! Many thanks. Need some time to go thru the articles...
Erm..1 more thing that I'm not sure as below. Say, now 12MP (NEF) gives us A3 print size (so called). Then s'ld I resample the image before, though the design size is A3? Or, just use the original will do?
P/s: NEF open in photoshop shown as 240dpi, am I correct ?