Most computer games require a powerful video card and the CPU doesn't matter that much as long as its fairly recent. It sounds like for civ 5 the video card won't matter as long as you have a dedicated card from the past several years, and the processor will need to be good. The rumor is that the processor will need to be dual core minimum. Turn based games don't usually have a minimum speed for CPU's, it just takes a long time between turns if your CPU sucks.