I had a similar system to the OP and it's just laggy. It's not worth upgrading as someone already mentioned, he has to buy video, mobo, cpu, etc so might as well get a whole new computer.
This game does not run well on any single core processor. Dual core cpu came out 3 years ago. The system requirements are vauge but multicore is a must. So most computers older than 3 years will not run this game well. Does this game have low system requirements? That's debatable as everyone has different standards. imho, this game has medium requirements (not too high, not too low).