Glest is a great 3D game with amazing graphics that will keep you busy for a lot of time. You will have to harvest crops for your army, you will have to create buildings for your towns and you will finally have to create the army that will destroy your opponent. The gameplay resembles that of WoW and Empire Earth, but at a simpler level.
The game is very customizable, not only in terms of gameplay options, but you can also customize units and buildings using the editor that is included within the game. Because the level of detail is so high, you might even have to tune down the settings a little bit if your PC is older.