5000 lumens a sq. ft. will grow very well. Keep in mind that a 150w HPS or MH will put out around 13000-15000 lumens, and a 250w will be around 25,000 lumens. In other words, a 250 will do a terrific job in a 4 to 5 square foot area.

Having a ton of lights is okay if you have ventilation...but if (like most people) you have some problems there, don't overspend on lights. More light = more heat = more stress if you can't deal with the centing adequately. Get enough lights to do the job, use reflectors, and have good reflective surfaces (flat white paint or mylar) on the walls.