ESO = Elder Scrolls Online
Game is a little big of a pig, but not as bad as some others. High end specs can definitely make it run smoother and your definitely solidly in the mid end range.
Just playing at 1080p? Using stock medium type visual settings, higher, or custom?
I am sure there are some ESO performance tweak posts. Usually there is one or two things that make very little visual difference but improve performance greatly. Such as view distance, shadows, grass density, etc.
Your running your i5 at stock speeds, turbo is enabled? See no reason you shouldn't go up to at least 4 ghz.
To me your system is well past its half-life and heading into retirement years. Your probably better off selling the whole system off as a working decent machine and building a new one than upgrading a couple pieces and having what you took out sit around. Otherwise if you want to keep the guts: case, psu, ssds, etc are all still solid then start with video upgrade, then cpu\mb later. GTX 1070 once its out and initial price gouging is over with natch.