Perhaps what the problem is is that some Postscript fonts do not render well for screen use. I have set up Mozilla 1.7.3 to use Bitstream Charter and Bitstream Vera Sans, and it seems that they are picked up in antialiased Xft form by Mozilla. (On FC2).
If you look in the Gnome font browser (or the KDE equivalent) you will find that some fonts render far better than others. This is almost certainly down to the question of hinting in the original fonts, which usually improves screen display but makes little difference to printed output.
The scribus web site has an interesting piece on fonts at http://docs.scribus.net/index.php?lang=en&sm=setup&page=fonts2 , which points out some of these problems. ("Also, do not be put off by the lack of a great screen preview with the URW fonts - they are excellent printer fonts. Font faces like Palladio and Utopia for example, are not really attractive on screen, but they are excellent fonts for easy to read documents.")
R.