Welcome "Sir Anthony"
There's no way in-camera to force the file naming to lower case.
As far as photo management in linux goes, digiKam is what I used. I *think* there's an option in its settings to tell it to lowercase the file names as it downloads your RAW & JPG images from the camera/card. It will organize, keyword tag, assign default EXIF/IPTC data, etc.
For RAW processing and editing I used LightZone (lightcrafts.com). There are others - RawStudio, UFRaw for example - but I found LightZone to be the most comprehensive, intuitive and
compatible with the K20D color profile. It does cost $99 (used to be $199) but it's worth it for the peace of mind and functionality, in my opinion.