A 400w HPS would be great if you have a reasonable ventilation system planned. If you can just keep your closet door open when the light is on, that's the cheapest ventilation option. If the door needs to be closed then you'll have more work to do in that area.

And no, you had it reversed: more light during flowering, don't need so much light during veg. I'd just use some 6500K CFLs for veg, like two 42-watt CFLs per plant, or three 30-watt, somewhere around there.

Or check these lights out:

http://www.1000bulbs.com/product.php?product=31682

One of those per plant during veg, and if you can, use them with a 400w HPS for flowering too. I ordered a couple of those in the warmer version (4100K) for flowering an extra female plant I had laying around that I didn't want to kill off. They seem like a pretty decent deal in terms of cost-effectiveness.