Large Scale Smoke
Learn the secret to rendering realistic smoke in Blender Internal! It will be slow, but still faster than cycles.
Handy light cache comparison here.
And a density scale comparison here.
Remember: For large scale smoke, the higher you set the light cache, the better your result will look, and the longer it will take to render!
My settings for the billowing smoke seen on my tests channel can be found here.
For the animation I only used a light cache of 100, not 300.