I have 6x26w CFLs on 2 plants that are 4' each. Don't let yours get that tall as the lower branches of mine were not getting enough light, so I trimmed them quite a bit. You would get the same yield with shorter plants with the light reaching all of the plant.

I started with 2x13w CFLs and upgraded as they grew. I would not recommend anything less than 3x26w CFLs per plant, and if possible arrange them in a triangle around the plant so that all of the plant gets even light. Don't forget to have some kind of reflectors if you can get them to help from losing any of the light.

As was said before, keep the lights as close to the plants as you can w/o burning them. (usually about 6")