Aurora (sparc Fedora) 2.0 you can try... the BSDs, Debian... most of the others have fallen by the wayside... you are most likely going to run the E420R headless, so installation should be easier.
Solaris would be the best choice to start with... you would learn more about the box and how to get around... you still need to learn the "words" associated with Sun Sparc boxes...
haroldkarl