Majesty: The fantasy kingdom sim. One of my all time favourites. Imagine a mix between Dungeon Keeper, Warcraft & Dungeons & Dragons, that is Majesty: the fantasy kingdom sim. It can be found for a pittance on Gamer's Gate.
*edit*
http://se.gamersgate.com/DD-MAJESTY/majesty-gold-edition?caff=2620971 I'm a sneaky bugger, so I added my tag to the end of the link ;) (meaning that if you buy through it, I get a few blue coins)
MechCommander is another nice game. The Gold version should be very cheap. It is a TBT and not a TBS, but it deserves more attention than it usually gets.
Outpost 2 is also slightly different. Apart from combat, you have the whole colony building party, which is surprisingly complex. If you want something a bit older in a similar vein, then Utopia: The creation of a nation is interesting (though a bit clunky).
The old game Dark Omen is a 3d game, but it is not very demanding (it was released in 98). It can be a bit tricky to get to run on semi-modern computers, but there are several guides online that might help you.
Also, if you are prepared for something more complex, how about Paradox's older grand strategy titles? Europa Universalis 2 (don't get 1, it is objectively worse than 2), Victoria: An empire under the sun, Hearts of Iron and Crusader Kings are all very rewarding games to play.