I doubt the game will use much more memory than what you're seeing. The amount of data it stuffs into RAM isn't very much unless you're running a rig with 1GB or less. As for the one core of your processor being used while the others idle; I don't think the game supports multicore. It will run on one core but not be spread out among the others.
I could be wrong so don't take this as a defacto answer to your question. Someone with superior tech fu will be along to give you the definitive what's-what.
