As others have noted, buying more RAM is rarely a bad idea when working with images, particularly very large ones.
However, another way to approach the problem would be to design your poster in a vector graphics editor such as Adobe Illustrator or Inkscape. As vector graphics are scalable, they can be rendered at any size or resolution without suffering from pixelization. Also, the amount of memory needed to store and manipulate them depends only on the number and complexity of objects present, not on their size.
Obviously, if your poster includes "photographic elements", then those will have to be embedded as raster images. However, you won't need to scale the photographs above their natural resolution, since the vector graphics renderer will do that for you. (In some cases, you may want to scale the photos up by a relatively small factor, e.g. 2x or 3x, ideally using a high-quality rescaler like PhotoZoom Pro or Perfect Resize, but this depends a lot on how high their quality is to begin with — images with low resolution but high sharpness generally benefit most, while those with a high nominal resolution but low sharpness usually won't.)
It's also possible to trace raster images into vector outlines, but this tends to only work well for "cartoon-like" images that already consist of relatively clean and simple lines. For photos, you're generally better off sticking with raster images and only embedding them into a vector drawing.