I find these to be the best bet as far as most effective...
250W - 2X2
400W - 3X3
600W - 4X4
1000W - 5X5 or 6X6 at most
While a 1000W bulb will decently cover an 8X8 area for clones or vegging plants, it's important to remember that it might not be as effective for flowering ones. You've got to keep in mind that if the bulb is centered in a 8X8 area then that means the outer plants are only receiving something like 5-10,000 lumens. For best results on growing buds you want something like 80-100,000 lumens over the canopies. Just my $.02