You cannot "force" games (or any other software) to use more RAM than what it is written to use. If a program uses 1.5GB RAM, that's what it will use and the only thing that can change that is rewriting some of the code to use more.
There is a registry hack to allow 32bits processes to use up to 3GB RAM, I remember seeing it while trying a PS2 emulator that happened to require it. But again, the software needs to be written to use that much RAM for this to have any meaningful effect.
You can indirectly use more RAM by simply letting Windows use whatever spare RAM the system has as a file cache. No special action is required for that.