The recommended system requirements are a 2Ghz dual core and the Nvidia 7800GT. You could probably just get the AMD A10-5800K and forget the graphics card. That wouldn't be great for other games but should breeze through Sim City/The Sims and most RTS games. Should even be OK for FPS if you don't mind medium settings.
Not needing a graphics card means you could create a tiny system as well if you wanted.