Scott,
What you need depends on the resolution of the monitor. I use a 22inch monitor that is 1680 X 1050. At less than this resolution your performance will depend more on the CPU and less on the strength of your graphics card. At resolutions higher than this your frame rate will be more bound to the power of the graphics card.
Without knowing more about your situation I will recommend the following: For a CPU the intel core 2 duo E8400 and for a graphics card the ATI Radeon HD4850. These will run your game at nice frame rates without overheating. There are more powerful (and more expensive) options but it sounds like you don't need them. Hope this helps.
-jason
