Update: Patch released on 19th of November lowers RAM restriction to 4GB.
Minimum requirements clearly states, that the game needs 6 GB of RAM. These requirements are usually only recommendations and players got used to not follow them literary. CoD: Ghosts is taking completely different approach: it checks your system, and if you don't have 6 GB RAM or more - no game for you.
Someone would say it is one of the first titles for next gen consoles, so of course it needs a lot of memory. But is it really true? I tested the game and on my system it never got above 2.1 GB and pretty much stayed always at 2.1 GB regardless of video settings.
Here you can see, how main process uses system resources in time. Green graph follows CPU usage, yellow graph memory allocation and blue tracks HDD access. Graph captures the game from the start, level load (first rise to 2.1 GB) and loading new level (valley and second rise). As you can see, memory allocation is far from 6GB and even if OS and running applications overhead is taken into account, there is no way needs 6 GB.
This limit can be disabled by using cracked replacement executable file. It works only for singleplayer version and using it will probably get you VAC banned. Google RamFIX and the game name if you are brave enough.
Now, why would Infinity Ward implement and test this hard limitation when they got hands full with finishing four different versions to other platforms? And from ours and external observations there was no need for it at all?
Let me put on tin-foil hat and say that one reason could be to make next gen consoles more appealing: my last year gaming laptop won't even launch this game, but next gen is running it perfectly! Given that IW was given preferential treatment by Microsoft and Sony I would not be very surprised if that would be true.
More about CoD: Ghost hopefully in a few days in our Port Report or check fixes and guides for a few issues in our wiki article.