Q: Can you take a 72 ppi photo and make it look good at 300 ppi? Or, is it only used for taking a 300 ppi image and making huge posters?
A: My first thought (depending on what the image is of and whether I could get a higher res one easily -- because that would seem to be the better choice) would be to edit in an image editor (e.g., photoshop) , then save at a higher dpi, and then re-edit to fix any problems, smooth out jaggyness, fix white balance, color correct, etc. at the pixel level...
It does seem like Genuine Fractals, a Photoshop plugin does some of this.
It would still be chancey and it might be easier to either re-do the image (take a new photo or get the original and resize...)
I really have never heard of a good way to upsize, given that the pixels are just not there to begin with. If anyone knows of a way to do this, I'm interested -- although I shoot in a raw format and scan in TIF, just to avoid this issue.