For those using Photoshop, there are new options for upsampling images. Below is a link to the full article, but to summarize-

"Bicubic Smoother is a new interpolation method specifically designed for upsampling. As its name suggests, it gives a smoother result that handles subsequent sharpening better than Bicubic sampling."

Here's a link to the sample chapter "Understand How Color Works in Photoshop" from the book "Real World Adobe Photoshop CS2" written by Bruce Fraser and David Blatner- Adobe Press.