You have to remember however it's not just the amount of lumens that help your buds grow thicker. Florescent lighting contains a large spectrum of light that is for the most part non essential for the flowering stage of growth. If I were you I would save up some money and buy a HPS fixture. The Florescent lighting will work fine for the vegetative stage but by using a HPS bulb during the flowering stage you will get a bigger yield and an overall better final product.

Happy Smoking,
Skunkmaster