need advise!!lights and supplies

ok so i'm new. i got 1 run under my belt. next run i'm gonna step it up a lil bit. i have one 400w hps. my new area will be 2ft by8ft by 5.5 ft tall. my first ? is what light should i buy? another 400 or a 600w? the lights i've decided on r only like $10 different for the 600w but will this cause uneven growth?
also wheres a good place to buy this stuff? i will also need coco? thx in advance

Are you going to use both lights for all stages of growth? Or use one for flowering and one for veg? Also how many plants are you planning to grow? Sorry, your questions raise more questions heh.

I run two 400 Watters in a bathroom that I converted and I'm growing 8 plants...i like my 400s...im with homegrown though r u trying to veg with on then flower with one?

yes veg and flower for now. althou the lights im looking at come with both bulbs. i hope to do 5-6 plants.i did 3 last time but chopped one early so the other 2 got more light and air movement. i was happy with the results. my new spot is about 6 inchs wider and twice as long. i think 6 will b ok. in the future, both lights will b for flower only. but for now will only have the one room
also the new light will b a cooltube but my old light is just a cheapy metal reflector

o yeah lil more info, i use coco gro drain to waste in 3 gal pots.might add some perlite next run. i use gh flora series nutes at about 1/2 strength every other watering