http://www.cora.nwra.com/~orion/im/ke.ps
to jpeg and get decent lines and text quality:

Using a newer versions 6.5.1.2 and 6.5.8.10 on Fedora 11/12 I get lousy lines and text quality:

Other tests show png -> jpeg with poor quality, ppm -> jpeg look fine. Interestingly, with recent IM, convert uses the pngalpha device for gs conversion, and pnmraw with version 6.2.8.0.