i need help deciding

ok i have been fighting back and forth on this one and i am hoping that someone can give me an intelligent answer as i am at my wits end......
i have a 4 x 4 x 8 foot room which i am going to use as my flowering room.
I was going to simply purchase a 400 watter HPS and ballast kit for $109.99 brand new from e-bay.....will this size light generate enough lumens or what ever for that size of grow room?????? also another reason why i was going to buy a 400 watter instead of the 6oo watter is because i wanted to cut down on electicity as well and running a bigger light means running more fans and what not too right????
I guess my main question is in the long run, would it just be better to purchase a 600 w HPS w/ a digital ballast
now or should i just purchase the 400 watter ????
my goal is to keep a constant flow of herb available for personal use only. nothting more nothing less...plz help thnx.....

i like my 400 but if i could do it over again i would get the bigger one, the difference in the two arn't that big as far as heat, and as for electricity the 600 i think is supposed to be the overall most efficient, so more bang for your buck, but a 400 should do that room just fine invest in mylar for you walls and you'd be golden.

thnx dr. i have been online over there at Cannabis and Marijuana seeds :: HomeGrown420 - powered by vBulletin and well they too over there are telling me to go with the 600 w HPS. so i have been searching all night and all day and what i found to be my best bet is going with the 600 w HPS w/ digital ballast w/ memory chip and a easy cool 6 hood w/ a 6" hook up for hoses. I think i will be purchasing these items from "High Tech Garden Supply". Has anybody ever gone through these guys and if they have, are they any good????? I was refered to a guy by the name of tony.........plz if anyone knows anyhting about this positive or negative let me know .............. i'm tired of being ripped fucking off!!!!!! thanx!

htg has a pretty good rep, never bought anything from them just cause i refuse to wait for shipping so i always just end up going to the hydro store nearbye. but i've never heard anything about any memory chips for hps lights? whats the chip for?