Call of Duty: Ghosts PC apparently uses 2GB RAM at maximum settings, users create MP FOV unlocker
Call of Duty: Ghosts is one of the first big budget PC games to demand its users have more than 4GB of system RAM, so much so that the game simply refuses to launch if it detects there's less available. However, users on Guru3D and other forums are reporting the game only occupies between 1.5 and 2GB of RAM on the game's highest settings. Additionally, unlike Treyarch's Black Ops 2, Ghosts is locked to a 65 degree field of view, which can lead to motion sickness among some users. Luckily, some modders have created a utility that will unlock the FOV in the game's Multiplayer to help you avoid some headaches.
Of course, I'm no technical knowledge man, so observing what the Task Manager reports might not in fact be a useful metric. Some users have raised the argument that the RAM might be used by a file buffer that isn't reported by the Task Manager.
Additionally, these reports don't necessarily cover the game's entirety, there might very well be a particular section that requires vastly more RAM.
We've got a full Call of Duty: Ghost campaign walkthrough guide here, with extra multiplayer tips.