Unless you're planning on truly large plants, 600w isn't just enough...it's overkill. A small plant takes up 1 to 1.2 sq. ft. A medium plant takes up 1.2 to 1.4 sq. ft. In other words, 6 sq. ft.--an area of about 30" x 30"--will grow you four good sized plants. We're talking plants about 30-36" in height, the kind you get if you start flowering at around 12" to 15". A 250w will actually handle that pretty well, although most people go for a 400w.

If you flowered longer--say 6 or 7 weeks--you might get plants that are over 4" tall. Say you gave each plant almost two feet to grow in; 22"x22" for each plant. That's around 13 sq. ft. You'd definitely want a 400w for that. But here's the deal. At that height, you start to lose light penetration, even from an HPS...even from a 600w HPS. If you've got the overall space and the ventilation, a 600w will do a great job on four really big plants. But if there's any issues with size or heat or ventilation, you might want to scale back. Like I said, even a 250w will take care of four medium sized plants.

And that's for flower. Big lights are kind of a shock for small plants just starting out. You're better off starting under CFLs for a couple of weeks, then switching over to an HID light. I have one other thing that I consider important. When you switch to the HID light, keep the light a good 6-8 inches higher than you need to have it for the first day or two. Lower over the next couple of days after that so the plant gets used to the increased light.