Okay I've been living with a boarder line PC for a long time now and I know how to squeeze out every bit of power the thing has so I can give you some tips on how to increase the smooth play of your game.
First of all, this may be obvious to some but not to all. Mak sure you're not running a virus scanning program or spyware program ect in the background, it is a single player game so you don't need to be on the net to play it and you won't be downloading anything ect whilst you're on it. Secondly hit alt+ctrl+delete to look at your programs manager and stop anything that is running that you don't need for example Punkbuster or ipodservices or adobeacroboatereader , these things tend to run in the background even when you're not using them taking up space. If you don't know what something does, don't stop it, you should still be able to hammer down a lot of stuff like this.
Secondly if you're using windows XP there is a little trick I use to free up a lot of processing power. First launch Galatic Civ 2 so it gives you the menu asking you if you want to play or go to the website ect, the launcer screen. Then hit Alt ctrl Delete, and stop windows explorer. This will cause all the interface for windows to go away but it won't crash your computer. What it will do is free up a ton of memory that windows uses to run. Next close the task manager so that all you have left on your screen is the launch window for GalCiv 2. launch the game and see if all these tips have made your game smoother. If not I'm not sure what else to tell you 
