Fire totems do indeed receive +dmg and +spell crit from your gear, as of a few patches ago. It even gets the x2 talented crit bonus.
I heard the damage coefficient on nova was something like 20-25%, but I've not personally tried it. In any case, it'll never be our go-to damage dealer (unless you're doing something funky like a Tankadin AoE run), so you're better off concentrating on improving Lightning Bolt damage. As you improve your +dmg and +spell crit, nova will get better.