[quote name='Genocidal']Civ IV is cheaper, has a lot of good mods, and a lot lower system requirements -- I've heard Civ V can get laggy towards the end of games because of all the calculations but have never played the game. I'd pick up Civ IV the next time it's on sale, and then if that's not enough for you go for Civ V.[/QUOTE]
My first playthrough of Civ V, I really noticed the slugglishness, even on my Dual-Core MacBook Pro (which has been more than adequate for most other games). It was also pretty buggy, crashing after 5 or 10 turns, so I had to increase my auto-save frequency to avoid losing all of my work. I know it was updated by Steam since I first played and that may have improved some of the problems, but I haven't tried it again yet so I can't say one way or the other.
I also have Civ III and Civ IV (another Steam sale) but I haven't played them yet. Might give III a shot next before trying V again.