Hey Kaze,
I don't have any fans yet... I will buy them a little in the future. You're right about the airflow... just to replace the air inside the box and keep the temperature at optimum. As to the holes @ the top of the cabinet, yeah, that makes perfect sense.

The reason I'm going for two fans is::: I'm running a 4-5square feet space, with a 1000W system. It has one 600W HPS, and a 400W MH. I really doubt I'll run them both at the same time, but in anycase, 600W (or 1000W) is shitloads of heat to put out. That's the reason for the two fans.
In outputting the air, I'm really concerned about the smell... I don't want to wake up one morning and find out my entire crop's smell is about to get me busted. So, I'm putting a 6" exhaust tube, connected to an activated carbon filter ( http://www.overgrow.com/growfaq/1509 ). I hope to be able to make my cabinet sturdy enough so I can seal it as much as possible... I'll be using that caulkin silicone [?] and some weather strips.

My cabinet isn't in a family place... it's, actually it will be built inside a closet. Nobody will see it, except me... and you guys!

As to the strains I'll be growing: I ordered some White Russian seeds... I forgot from where (! stoner!!!)... I guess they're still about 3-4weeks away... if they don't get stoped by customs... that would suck dick. I don't know what the fuck I would do then. I'll ask the guy to re-send the seeds. I guess he'll be understanding.

Keep those updates coming!

-turtle420