teshra: What happened to Expeditions: Conquistador? I recall you being anxious to get it and then excited to play it and that wasn't that long ago. Its turn based with a historical story as the background. Imagine it meets all your other criteria too, unless the graphics card can't handle it. That may be the only hangup since I've read its a bit of a resource hog.
If not that, I like the idea of Colonization. Similar kind of game and the recommendation for it in a prior thread hits all its good points squarely on the head.
That is to play on my main computer at home and definitely won't run on my girlfriend's computer. I haven't even tried to install the graphics card yet(am hoping to not need it) as it is ancient and finding a driver will drive me nuts :P I am limited in what I can download at home as I run the net through my mobile phone (with a 5gb a month data limit) as I can't get fixed line internet where I live. That means I download games at my girlfriend's house and then install them at home. Unfortunately with Steam, that means sometimes the installation wipes the transferred files and starts to redownload it again, which is quite frustrating. From memory something happened with the attempted installation of Expeditions, but I'll check when I get home today as yes I am very keen to play that game :) The other issue I have with my main computer at home, is trying to get to use it. It's a power gaming PC and my girlfriend loves games and she somehow manages to get far more use of MY computer than I do :P
Edit: I own Expeditions on Steam, which rules out using it on the computer I mentioned :)