[Mylon]

Grab Alpha Centauri from a bargain bin. It's a very fine game by itself. Alien Crossfire has been deemed abandonware. Abandonware can be downloaded from some not-entirely-legal-but-hardly-prosecuted websites.

I played Alpha Centauri for countless hours by itself and I can say that the expansion didn't really add that much to the game. Granted, there's no point in not having it if you can get it, but it's not totally necessary.

Just be sure to have the latest patch. The only bug I've found while playing is that the game will potentially crash if an enemy attacks a base and one of your airplanes can scramble to defend. This bug was fixed, though, leaving the game in perfect shape.