If the only place you are lagging is the cities there isn't much you can do.

I would only suggest bumping up your RAM from 2GB to like 6GB or higher if you can afford it. You have a 64bit Operating system. So you can go much higher than 2GB. If you had a 32bit OS you would be limited to only 4GB which your not even at that point yet.

I've heard that SSD's help to when in cities to load player character data faster when they pop in.

But lastly we must just live with the fact that the servers are in Japan and it's a worldwide server. You can't do much with that unless your ISP is crappy.