world per city?

Only possible upgrade would be 16gb, or just good old fiver optic cables if it isn't running with them already. It's enough, there are just too many players online.
It's like trying to serve everyone one pot of stew at the same time, there isn't enough to go round. Lag will most likely be a factor you have to live with.

Doesn't matter if the cords from the server to the nearest hub is fiber optic, the difference would be negligible because most of the country still uses copper or gold I think.