...running under WinXP on modern hardware.

Online = any online capabilities, can be lan (hamachi) for example.

Civilization pretty much fulfills the economy-oriented requirement, but it is turn-based. And most real-time strategies are focusing on warfare, making economy at most an equal part, but I can't recall any game that would stress economy over warmongering.

I'm waiting for your suggestions and thanks in advance.

Heavily modded Oblivion (but without the latest add-on; I don't have patience to repair or play from the beginning if something happens).

Max Payne 2 with one of the total conversion mods. This game is perfect: not to hard, not to long, you can beat it in one afternoon and fully enjoy the great story behind. As well as shot some heads ^^

Armagetron Advanced - great, great thing, if you have a bit of time and want to just play. (Multi-player of course)

I was also checking out Kudos - nice thing, but I would prefer, if it was a game for cell phones or sth, that you can play in spare time.

I will (today or tomorrow) beat Fable:TLC again, as I want to check few things out.

And the last game: Supreme Commander. Unfortunately, my Athlon2600XP+@1,8GHz is not enough to fully enjoy SupCom :(