A space that size could easily house a 400 watt light, you might want to consider C02 since you plan on not using ventilation to enrich the air for your potential girls.
Or consider a few flourescent T5 fixture's for your area or a 400 watt HID with some flourescent's on the sides is always a good small space option.
I wouldn't spend the money on the 600 watt light, you'll just be stressed when the temps spike past 90 and you have too open the closet, turn off the light or buy a small A/C (not for a closet)
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like i said 10 foot x 4 foot closet 400 or 600w hps for 6-8 plants will it be to hot there is no ventilation and our climate is about 65 degrees outside.