If you have an air cooled hood... run that size ducting to an in-line fan... sorry, never actually done this, but it's round with the inlet and exhaust in a straight line... kinda looks wheel shaped...

For the carbon filter, I would prefer an in-duct fan. I'll try to find a pic, but you can get them in home dePot. It's basically a fan blade sitting inside the duct work. Pretty sweet, actually, and fairly cheap, I believe. Then you just make the filter, size on the DIY on here is about right for a cab grow, and run the ducting... and the fan is IN the duct work!!!!! I, personally, would make the ducting AFTER the fan route back into the room... filter the air, then put fresh air in the room instead of through the wall, ya know? It will definitely get you through the veg stage and would probably suffice during flower. Leave that fan running 24/7 if it has a low CFM... you can probably put a dimmer switch on it as well (Radioshack has potentiometers if you're on a tight budget, just check the specs on them). You may be able to throw it on a timer if you feel it's working well and don't need it on all the time.

I'm about to start the ventilation and HPS setup as well, so we can help each other out on this. Buddy up!

I have my yahoo userID posted. Feel free to add me! If not, I'll be more than glad to stick to the forums. This site is a blessing from God, and everyone on here is great, so long as you don't ask TOO noobish questions and show you've done some research.

Sorry about all the edits... just added little words here and there to make it make more sense!

Take it easy bro.