Of course there's only so much you can do to improve an image. You can't restore any details that aren't recorded in the original. But you can do things like change the contrast, remove noise, darken the text and/or lighten the background, and resize them to make them easier to read.
I'm no expert on such restoration, but I'd probably start with adjusting the gamma or contrast, and take a look at the various noise filters. Also get on google and search for tips on cleaning up text in images. You'll probably find some good info.
Finally though, while
gimp is good for processing individual images, it's going to be hard going to do hundreds of them. Once you figure out what you need to do, you should probably switch to the imagemagick
convert program and script out an automated filter to do the rest of them in bulk (I know that
gimp has a scripting engine too, but I'm not familiar with how to use it).
In fact, take a look at this great list of ready-made imagemagick scripts. There's even one called
textcleaner that looks like it does exactly what you want.
http://www.fmwconcepts.com/imagemagick/index.php
