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Photoshop would be the best tool for attempting this. I'd experiment with the bicubic interpolation settings increasing the image at 10% intervals. In CS3 there is a 'bicubic smoother' setting that is designed for increasing an image's size. Then you can tweak areas of the image using sharpening and blur tools to clean up areas of pixelation.
To make an image smaller is child's play - open it in you favourite editor and typically choose edit/image > image size > then select the %/dimensions you wish to resize it too > click go :)
But, as limbo said, there are ways to make the best of it. Also, bear in mind that each time you make *any* adjustment to a jpeg and then save it, you lose image quality. It may be worth saving as another lossless file format, such as tiff or EPS.
[edited by: Grandmas_Cookies at 12:36 pm (utc) on Oct. 8, 2007]
Another method is to scale them in steps using bi-cubic. Use Neat Image in between steps to clean out the fuzziness.