26w cfls are the most efficient. 42w are less efficient, but you can use less of them (but they are more expensive). For vegging you want daylight spectrum (6500k) and for flower you want warm white (2700k).
I use 26w for vegging. I also combine 3 socket y-splitters to create a 4-bulb fixture that I hang over my plants. I use a plug-to-socket adapter on the end of an extension cord into which I screw my fixture. Cheap, easy and effective.
For just one or two plants, you could probably use 2 42w per plant or 4 26w per plant. I like using multiple 26w bulbs because you can arrange them around the plant to provide more even lighting from more angles.
If $$$ is an issue, stick with the 26w. You can get a 4-pack from a hardware store for around $12. 42w single pack bulbs start at like $8.
As far as electrical usage...4-26w bulbs = 104 watts...slightly more than a 100w incandescent bulb...remember when we used those things?
