@orielwen: Don't upsample. Ever. You don't add any information to the image by upsampling, you just make it bigger. (There's a standing joke in the Photoshop community about the “CSI Filter”not having been invented yet. That's the one you see on TV, where some bloke with a computer takes a vague blob in one corner of the photo and turns it into a sharp image of the suspect.) Final output quality of a sharp lower resolution image is just as good (better, in most cases), than from a upsampled version of the same image, because “upsampling” is really just some algorithm filling in its best guess as to what those extra pixels might have been. It's not the real image data, the RIP has no way of knowing which pixels are original and which are bogus when it builds its half-tone screens, and you have no control over which ones it decides to keep and throw away in the process.
I do quite a bit of theater and concert promotion work, so I've become quite used to images that are way below what conventional wisdom says are minimum size/resolution, and to how good they can look in print if the contrast and tonal range are optimized.
