Civilization V is pretty much a staple for any PC I can imagine. It's just a solid game. Galactic a Civilizations II is not quite the necessity, but it's an extremely fun title. I haven't played the 3rd one or I'd recommend for or against it, but I remember it getting good reviews. I'd recommend Star Craft II if you're looking into a good competitive RTS; I'd also recommend Sins of a Solar Empire for a much more laid back RTS that takes longer.