Just my opinion, but if it's your first grow. I'd seriously consider using only a part of your closet and growing fewer seeds. You could do a CFL grow with 4-6 seeds, and get 2 plants. It would be cheap, and you'd learn a ton about watering, using fertilizer, transplanting, and flowering. Put it this way...get four 10.5" reflector dishes at Home Depot or Wal-Mart for $10-12 a pop. Get 4 Y-adaptors at Wal-Mart for $1.50 each. Get 8 23-26 watt Warm White (2600k) and 8 23-26 watt Cool White/Daylight (5000-6500k) for about $30-40. That's less than $100 for enough light to grow two good plants in a small (14" x 28") area. You're much better off making mistakes and learning about how to do things and what works well in your closet with 5 or 6 plants than with 20.

If you've got that much height, you'll probably be able to use up to 250w of HPS or MH with minimal ventilation (an occasionally open door and a couple of fans might do it). There's a lot of room for heat to dissipate and that's good. But a 250 covers around 5-6 sq. ft. .... a little more than half of your space. Like I said, you might want to look into T5 lights. I've never used them, personally, but I've heard some good things. A four bulb set up with 4' tubes would be a perfect fit in your room. I'd search and see what others have to say.

Still, I'd really think about starting out smaller. Don't blow all your seeds on your first grow! Good luck!