Your other games will be gathering dust on the shelf once you break out Civ IV.
The games has all the additctive traits of its predecessors and now has added much better graphics, animations, more world variety and a better combat system. I like the changes to the spies (they can't subvert cities anymore), the new improvements, religion's new importance, the 'tailoring' of your civics and making pollution something you can't just clean up with a Worker: it's an internal problem now and a challenge later in the game. You will not be disappointed. The only reason I did not rate this game a 10 was because of a noticible sluggishness (even with the latest patch) in the larger maps.