It'll be pretty tough to fit 8 full-sized plants under a single 400w system, and you'll most likely end up with some pretty wispy buds from the plants on the edge of the light footprint. A 600w-1000w would be more efficient for normal sized plants. (2-4 feet) If plants are kept short and the canopy even, like with a SOG, it'll be much easier to make the best use of a 400w light. Just remember to keeping things properly ventilated and under 80 degrees Fahrenheit.

Also, I haven't been to the site in a while, but I've found some really good lighting deals from a site called "Discount-Hydroponics". May be worth looking into. And don't forget to check places like eBay, Craig's List, etc., for great deals on used/new systems.