For lovers of SMAC, there is a total conversion of Civilization IV (with BtS) to Alpha Centauri called Planetfall.
You can find it at
http://forums.civfanatics.com/forumdisplay.php?f=278
It captures most of the originals features, except the Unit Builder. The graphics are an excellent blend between the old SMAC and the modern civilization. You can even copy the sound files from your copy of SMAC to the mod folder and have the technology voice-overs.
I'll second the recommendation for this mod. The people who made it did a great job taking advantage of Civ 4's features while keeping the spirit of the original game.
The Flowering index and Planet attitude towards factions was a particularly nice touch. Those did a better job of making the native ecology feel like a force you had to contend with... and letting you see how much all the factions were affecting it. Unlike the original, you can also have all the original and expansion factions in a single game. And Civ 4's combat system is much less asinine. In the original AC, I could never get over how your unit's weapon doesn't matter when you're defending, or your armor when you're attacking.
There are a few things absent from the original that people complain about, but I didn't really miss. No unit editor, but in the original it was tedious manually creating units EVERY time my tech improved (or pruning all the worthless/redundant units the AI creates if you had automated design). No alien factions, either, but I can see why they didn't bother with that. I never really liked the story behind them. It would've been more interesting if the aliens didn't have the exact same crash-land-start-from-nothing setup as the human factions.
I haven't tried every faction yet, but so far my only complaints about the mod are that the civics aren't totally balanced (Fundamentalism in particular is too powerful, and it doesn't even nerf your Science output like in the original), a bug or two remains despite how development seems to have stalled/ended, and there is no cash bonus for hunting down native life! But those are rather minor issues overall. People who loved the original Alpha Centauri need to try this mod.
Amazing job. I love Civ 4 BTS (my favorite civ by far). I loved SMAC. Seems to me that I must have a look at this
Rats, just now that I'm going away from my CIV capable desktop for three week.
I will try it as soon as I'm back.