it all depends on the source material... period.... end of story...
In my oppinion the best way to tell what resolution you should scan at, is at what point can you see the individual
print dots. Once you make the individual print dots the right size, you only make things worse when you scan larger.
around 150dpi for newspaper print, around300 for magazines, and around 600 for art books, for expensive artbooks i go
between 600-1200 dip.
The point is, if you go stupidly high, your scanner will scan one little ink dot to cover a stupid size of say 50 pixels
on the screen at 1:1, At this point its rather silly because your not getting sharper images, your just wasting space.
a good rule of thumb is.... make 1 print dot cover 2-4 pixles, at that point, you can do your best image manipulation,
cleaning, removing of scratches, removing text, etc....
when your done, use your gausian blur, smart blur's and any other filters you want to throw at it, and you will
have amazing results.
When it comes to actually scanning photographs, and negatives, ... there is no limit,
i once scanned a 1 inch by 2 inch black and white photo from the 40's for a guy who wanted to see what a scanner
was capable of, I asked him if he had any wallet photos... he said he did, and handed me this folded picture of a white
triangle with a black background, and it had blue hand writeing on it that had blead from a peice of paper onto
it.
scanning it in at 2300 dpi, i was able to not only turn the white triangle into his wife in her wedding gown, i was
able to remove the fold, clean up all the little scratches, remove the blue ink by just keeping the red layer, and
turn the black background into the wedding hall where he got married... printed it out 8½ by 11 on a laser jet
printer... and it looked like the photo was taken yesterday.
So once again, ... it all depends on your source material.