What is the latest Rockbox revision? if I upgrade, will I lose the material I have saved on the unit? I mostly am using it as a flac player (which is awesome)
Nope, you don't lose any files, you simply unzip over existing files as far as I recall. Check www.rockbox.org for latest versions.
indeed, just unzip and you're done. Settings are not changed so if you want (for example) the default theme of 3.0 you need to select it yourself...
NOTE: wise people have backups regardless of updating sw...