I maybe wrong on this one, but I find scanned prints more sensitive to noise reduction filters than pictures from my digital camera. Meaning it is easier to lose details when applying noise reduction to a print than to a digital photo.
I have not tried Neat Image, so I cannot comment on that one. I tried Noise Ninja once, and was not impressed with the result. Most of the noise you get rid off will return later upon sharpening.
Usually, I would use Photoshop's own noise reduction filter. I know that many people don't like it, but with the right settings, I would usually get the results I am looking for.
I would usually go with:
Strength - depends on the picture, usually lower values with scanned prints
Preserve Details - ~25%
Reduce Color Noise - ~40%
Sharpen Details - 0% (I would do that later using USM
Remove JPEG artifacts - unchecked
(I use a German version of CS2, so sorry if English terms are not fully correct)
For grainy skies, you can apply noise reduction only to the blue color channel.
Dirt / dust spots, I would clone out manually with the healing brush. Of course, it is also very important to carefully clean both the print and the scanner before scanning.
As for downsizing, I am no friend of incremental downsizing. I would usually downsize to the final size in only 1 step. But that's just personal preference.
My workflow with scanning is usually:
- clean print and scanner
- scan with max. dpi and color depth (48 bit) into a PSD file
- level / crop
- clean from dirt, dust and noise
- save the result as PSD (helpful if you would like to repeat one of the following steps with different settings later on, as cleaning is usually what will consume most of the time)
- adjust color / exposure / saturation / contrast
- downsize (1 step)
- sharpen (sometimes I would apply some USM
even before the downsizing - depends on the print)
- convert to 8 bit & save as JPEG