they use a simple trick called area based texturing.

this simply means if you have a house that needs a wall texture, a part for the wall to celing section and a part for the ground to wall section you dont create three different textures but you use one larer textured splitted into 3 areas.
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top area for the wall-roof connection
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the standard wall texture that is repeatable


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area of the ground to wall texture
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now if your house is 3 or more floors high you can simply use the lower 4th of the texture for the ground near areas, the top 4th for the roof areas and the middle can be placed tileable into all 3 or more walls of the different floors.

Its like unwraping models made out of wed geometry.


another point is detail mapping for the textures as well as bumpmapping...this adds lots of detail to the textures.

Others then that they dont use any tricks.
And you can use 512/512 textures ourdays...its pretty common. I know hardly any card that cant diplay it...sure not the fastest version of a texture but a lot more detailed.

hope that helps