not being a programmer, I cannot really provide details on my ideas but artisticly, it might help.
Depending on how you want to use your sky, a sphere might work but there is a way to reduce the amount of data i think, Divide your sky up into the different views.
commonly you might want to make 2 or 3 sub divisions. first, create a horizon graphic or face. this cill extend verticly from the edges of your maps. this can be many triangles but it will pay off. once you have done that connect those verticies to the others by making a dome like setup over the map. now the tricky par tis in the textureing.I would make more than one dome, each one a little furthur down from the last. each dome would have a random texture placement engine that would place a random cloud image at a random point on a random dome. the upermost dome is the defaut sky color and the cascading domes would be textured with clouds. since its a dome, you could run a animation loop that moves the images at differing speeds to get that nice current effect... this is a little hard to explain right so ill just have to assume you know what i mean. essentially, for the best effect, you will want a library of possible cloud bmp files. the you dont have to worry about file size so much.
If you are using 3dfx you could play areound with fog effects. this way you would not need to even make a sky object as the fog gets harder to see through as you go up.

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