Is episode 78, the last episode in Sonic X? I really want to watch more!

if its the 1 were chris leaves mobious then yes

Yes.. I did saw a commercial for Sonic X where there was going to be a new season, but I don't think it will happen.

Yippers, Sonic x sadly has only 78 episodes, and no following shows are even planed at all. We can expect no sonic tv for a LOOONG time. HOWEVER new games are planed for 2013 I belive, and if you REEEALY want more sonic x theres a list of things to watch.
#1 the sonic satAM show from 1993.
#2 the japanise sonic x. (has deleted scenes, and fun movies during the credits.)
#3 Some people like "sonic underground", however in my opinion, its not even close to sonic x.
#4 Watch sonic x in reverse, or upsidown. LOL

im afraid yes it's that's it

let's hope the director make season 2