Without hacking a 32bit Windows (/3GB switch in boot.ini), a single Process can just allocate 2GB max. Usually it's even less, somewhere around 1,3-1,5GB. Never have seen a game with more RAM usage except a MMO with a nice memory leak growing up to 3,6GB (on a 64bit system). Could have been more, I just stopped raising it
64bit Systems are up to 6 years old, it's time to get some 64bit support nevertheless