Hi, it took me hours to sort out this info, I hope this helps you:
Newbie 0-29
Jr 30 - 59
Member 60 - 119
Full Member 120 - 239 [Avatar]
Sr Member 240 - 479
Hero Member 480 - 959
Legendary 960+
In regard to the benefits I'm not quite sure, only know that Full Members can have avatars.
I think some of this might be outdated? Ive got more than 30 but am still a newbie
Edit: must not be post count but activity instead. Sorry