There are many threads on screen protectors. The most popular opinion is that the glass is indeed scratch resistant but not scratch proof (sand and other particles can scratch glass). Also the oleophobic coating wears during normal use as stated in the manual. So better safe than sorry.
The most recomended screen protector are the static cling kind, they look clear and won't affect the sensitivity of the screen. Any quality one will do: Power Support, SGP, Switcheasy, etc.
As for a clear case, it depends on the level of protection and material you want.
TPU: Switcheasy Vulcan, Casemate Gelli, iFrogs SoftGloss.
Hard plastic: Incipio Feather, Switcheasy Nude, Speck SeeThru.