Depends on what you are doing, if you have several sequences open, lots of footage, then Final Cut will be eating away your ram. Also when trying to play back stuff in RT, the ram will start kicking in.
Don't worry about ram usage, when it needs it, it will use it.
Final Cut Pro uses a maximum of 2.5 gigs of ram for its self because it is a 32 bit app and that's just what Apple says.