re: TBS games
Once I discovered Alpha Cen wouldn't play on a P4, I went to Gamespot looking for free demos to download, and pulled both Civ III and GalCiv. The Civ III demo would only play once -- when I tried to resume the game, it sent me to a screen to buy the full version instead. Bah! Uninstall...
And, believe it or not, I haven't had time to play Gal Civ for more than a couple of hours the same day I installed it... but it looked intricate enough to serve as a worthy replacement for Alpha Cen. As you like it, too, that's strong recommendation.