4GB is adequate, It's rare a game will actually make use of much more than 2GB (+ 1.5GB+ for the OS) as they are coded for 32 bit compatibility and anything more causes problems.
Nowadays there are a few games that do like more than 4GB. Thief, Battlefield, Starcraft II, but they can be played comfortably with 4GB.
Call of Duty Ghosts says it requires 6GB, and will be jerky with less but it has been proven that a large portion of that memory usage actually has no data in it so the developers have filled it perhaps to stop people from cheating.
If you are building a computer then It is wise to go with 8GB for the future as the new consoles both have 8GB and a lot of games will be designed with this in mind now that 64 Bit operating systems are pretty much common place.