help with constructing first setup

Hey all, been lurking a while, will now have my card and want to get my space constructed..
I drew a quick sketch of my space

http://i56.tinypic.com/fu75o5.jpg

The plan is to divide the space in half, so 2 spaces about 40-41" wide with a divider in the middle. I will create a raised floor so there will be cubby space for various things, we will also create a lower ceiling a little bit. I was going to line the entire thing with b&w poly and have a 400w on one side and a 250w on the other side.
If we created an edge along the openings, could I just run strips of velcro and then pull the flap open as far as needed?

As for ventilation, we were thinking we would drill 2 holes in the ceiling and run ducting that would connect above as a Y and then connect that into the closest bathroom ceiling fan duct. My local craigslist has 2 4" Vortex in line fans for sale, I found online these are rated at 172 cfm, is that going to be strong enough to keep the temps from being too high? There would be 1 of these on either side with carbon filters attached to each one. I was planning to have fans at the bottom moving air around as well..
Mostly just worried about it being too hot in there, not sure how much 172cfm is, any help/comments greatly appreciated.

this looks to be about the size of a closet... or are you building a box? if its a box then those fans will work decent enough as long as:

1. you seal your space to be light AND air tight, pandafilm and duct tape do wonders.
2. the air outside of your spaces is cooler than the air inside.

seal your space up, turn on your fans and see if you have negative pressure. then create a passive intake, start with a small hole, use a lighter and see if the flame goes into the hole. as long as the flame is sucked into the hole and not blown out, you are in good shape. if the flame goes out make the hole a little bigger and retest. once you have good negative pressure, all you gotta do is cool the area outside of your grow area until your temps reach nominal ranges inside the grow area.

if its not a box and its a closet, i would get bigger fans more in the 300cfm rating. but regardless you will still need cool fresh air supplying the area to keep it cool.

It is actually a closet, so I need something 300cfm or higher? Not sure how I am going to have air intake, I was going to put the closet doors back on when done, just cut holes in the poly like you said and then when i shut the door it will be light proof and still get air?
Any recommendations where I should get my fans? When I look at lowes.com or homedepot.com they only have booster fans so I guess I'd have to get them online, probably cheaper that way anyway..

I was also wondering about bulbs, when I went to some of the local shops they had all different brands of bulbs ranging in price from like 25$ to like 50$ for the 400w, any brand I should get or not get or look out for??