I'm sure this has been posted about before, but it's one of my favorite things to do, so i thought i'd share how i do it in case others would like to do the same.
The 2 games i'm talking about are Civilization 4 and Galactic Civilizations 2. I love combining the games back to back for an extended experience. For example i start out playing Civilzation 4, a good long marathon game. Starting out in the most ancient of times, building my civilzation on my CivEarth planet (whatever map that may be). I only allow space race victory to be enabled, thus u have to make earth's first colony ship to win, that same first colony ship that's floating in space a the start of a GalCiv2 game.
Once i've battled my way through my CivEarth's history, gameplay that can take a week or more, i ultimately come to the end of that game with the building of my first space colony ship. This is where Civilzation 4 ends for you, but thanks to GalCiv2, you can continue out into the galaxy!

What i do next is take a screenshot of my Civ4 planet map, something like this:

Then open up PaintShopPro(tm) or Photoshop(tm) and make the colors a little more vibrant, smooth it out some, and add some polar ice caps. I'll use the new image to replace earth.png and put it in the mods folder:

Then i'll transform that image into a smaller greyscale image to be used for the height map. Make the oceans darker, the land lighter and i get a height map like this:

Then i'll make a nice new colorscheme for my new CivEarth, so that the colors appear close to my new planet texture, and badda bing! My CivEarth planet from my Civilization 4 game is now Earth in my GalCiv2 game:

Now i'm all set to continue my journey in Galactic Civilizations 2! I've even made my own logo's based on my Civ4 culture, snapshots of my leaderhead, etc... etc...
It's a nifty way to have that caveman to insterstellar empire gaming experience so many of us want. It's not all in one game yet (it will be someday), but in the meantime, what a way to combine 2 great games for the ultimate experience!