There are a couple of important concepts you need to know when printing at this size.
The first is that most people don't put their noses a couple of inches away from a poster this size like they would a 4" x 6" or even an 8" x 10" print. They tend to stand back a bit so they can take the whole thing in. This means that you can get away with designing the non vector based part of the image (...I'll call it the "photographic part") to print at a much lower PPI (pixels per inch) than you are used to. For small prints, one typically wants to have enough *real pixels* so that you can print it at 250 or higher PPI. For a print the size of a movie poster, one can typically get away with 75 to 100 PPI, and depending on the amount of important detail in the photographic part, sometimes even lower. This will dramatically ease the pain and whirring your computer is going through, LOL.
BTW, I emphasized the phrase, "real pixels" in the last paragraph to distinguish them from "fake" pixels which have been artificially generated by some up-rez'ing process.
The second important concept to recognize is that even when the photographic part of the overall image can be much softer (i.e., lower PPI) than usual, people still expect to see very sharp lettering, and sharpness to edges in the pure graphics design part of the image. This typically means that the graphics part of the image must be effectively printed at least at 200 PPI.
The way the above two concepts influence one's workflow is that designers usually create the photographic part of the overall image in PS, but they will almost always create the text and other graphics design aspects using either the vector part of PS (if it is relatively simple) or AI, Adobe Illustrator, if it is complex. Then, at the last step, they will integrate the two together (typically in AI) and output the result to a file format that keeps the two parts of the image separate, eg, *.ai, *.pdf, *.eps, etc. and that their printer will accept. If you rasterize everything at 200 or 300 PPI, you'll need a very large computer and the printer will likely be annoyed by you submitting a file size that large.
With respect to your question about adding some grain to the image, this is almost always done as part of the pixel-based part (the "photographic" part) of the overall design, and follows the PPI recommendations for that part given above. If you don't know how to get good looking grain (...no, it's not just using the "add noise" tool in PS and blurring it a bit...), either use a plugin like Imagenomics "Real Grain", or similar 3rd part solutions from Nik or OnOne software, or just go to Google Images and search for "photographic grain" and only accept images larger than about 4000 px on the longer edge.
HTH,
Tom M
