Dealing with "digital" images is often a little different than traditional "photographic" images. Often there is more color correction needed.

As for the UNSHARP MASK its name is a bit misleading (you might think it would "blur" the object). It is a holdover from the old darkroom days, and actually sharpens the look of the image.

The "amount" slider determines how much to increase the contrast of pixels. For high-res printed images, something between 150% and 200% is a good setting.

The "radius" slider determines the number of pixels surrounding the edge pixels that affect the sharpening. For high-res, a setting between 1 and 2 is usually good (a lower value sharpens only the edge pixels).

The "threshold" slider determines how different the sharpened pixels must be from the surrounding area before they are considered edge pixels and sharpened by the filter. Experiment with values between 2 and 20 (default value 0 sharpens all pixels in the image). Another good "sharpening" technique to use is the Lightness channel of LAB mode (if interested i'll explain later).

As for resolution, the quality of your piece will depend not only on resolution of your scan, but also on the resolution of your output device (printer) AND the size of the original image as it compares to the final image output.
(This is often an issue when needing to scan from slides)

NOW FOR SOME MATH:
Multiply the screen frequency by 2 (the typical ratio of image resolution to screen frequency needed to produce a high-quality image - check with your printer...).

Then multiply the result by the ratio of final image dimensions to original dimensions to determine an estimated scan resolution.

Say you have a 1 X 1-1/2 slide that you need to scan and you need to ouput it to a 6 X 9 with a screen frequency of 133lpi (lines per inch). Multiply 133(lpi - the screen frequency) by 2 to get 266. Then multiply 266 by 6 (this is the ratio of final output dimensions to original slides dimension) to get a scan resolution of 1596ppi (ppi - points per inch). Check with your print specs in regard to the screen frequency needed...

Hope this helps,
:)
retrocity