you can do 400hps in something that size but you need to make sure you have good ventilation. i have a 2x4x8 closet that has a 250w hps, and it works great though. at times i wish i had the extra wattage but i am not complaining and my last crop of 6 plants gave me about 7.5z's fully cured, so i dont think they were really complaining wither...in any event, make sure you have god ventilation if you are using HID.
as far as strain, anything not mostly sativa will be good