my system with BF3 on all low, and nothing else running except for chrome, uses about 4.5gb of ram.
i had 4gb ram before, but stupid win7 task manager always showed 3.3gb used only, and which i later found out was not accurate since i was wondering why still so slow if still got enough ram.
after i put 2gb more, then can see it was actually up to 4.5gb...means before the excess was eating into my system pagefile slowing the system down.
so yea, in short go get 6gb ram.