The game must be designed to make use of two cores. Many modern games can use two cores, but there are still some modern games that only use one core. Based on some research the Sims 3 only uses one core. I play Cities XL 2012 and I know for a fact it only uses one core, the other 3 cores in my quad core CPU are not used by the game.
You only have 1.59GB of usable RAM because you have an integrated graphics core (iGPU). iGPUs are designed to use system RAM. iGPU might have some dedicated RAM like 32MB, 64MB or 128MB, but they basically rely on system RAM to keep costs down. That is why you do not have full access to the 2GB of RAM.
If you want more RAM, then you need to install it... assuming there is an empty RAM slot.