The game is running as fast as it can and using all the memory it can. Its not waiting on cpu or ram. When those go up towards 100% then you are approaching trouble and will need to upgrade.
its likely aion is waiting on info delivered over the internet, like what those players are wearing, where they moved to, what action they are doing... all so it can look good to you. Your FPS goes up outside of populated areas because now the game doesnt have to send you much info. All the map & terrain info comes off your HDD (as do all the graphics for what everyone is wearing, spell effects, & such)
You can try turning off AA type settings (maybe its called ssao) in the game graphics if thre are any but with a 2gb GPU unless your running multiple monitors or high resolutions, your 660 should be fine, just keep in mind that Aion is way more demanding than something like Wow.