Veg doesn't demand a lot of light thankfully. The more light you add, the faster it will grow, plain and simple. That said, growing slowly isn't bad for the plant, it just takes a little more time to get big. There are no real watt requirements, however most people tend to think about 50W a square foot is a good rule of thumb for full size plants. If you use a scrog or sog, it can be less.
Metal Halide is the most efficient lumens / watt followed by T5 fluorescent bulbs. Regular fluorescents are cheap initially, however they'll eventually cost more in electricity.