670 watts is more than enough to veg and flower 2 plants so if your light really is equivalent to that, you should be just fine!
My first indoor grow was in a closet with a 600w HID and 5 plants, worked great.
Make sure you have a game plan for the containers...are you going to re-pot at some point or toss them in big pots to begin with? I would (and do) start with root plugs (the pre-moistened kind) upside down, put seed in there in humidity dome ... Then to plastic cup with good light soil (I use Happy Frog) ... Then to 1 gallon pots (more Happy Frog) ... Then to 3 or 5 gallon pots depending on how big I want the plant to end up (FFOF is the soil used this time). While I'm transplanting into that pot, I take clones if applicable and do all my pre-flower pruning. Then veg for another week or two to recover, and then flower.
Point being, have your game plan for the whole grow in place ahead of time. For your closet grow, I'd say probably 2-3 gallon pots would be as big as you'll need.
Passive intake should be fine and any small circulating fan will do in there, but what about odor control? If it's going to be a concern for you, you'll need a carbon filter or some way to kill that smell.
And of course nutrients ... To include water ... Make sure your water is free from chlorine and chloramines. Get a filter or let it evaporate, whatever makes you feel good. I filter. Are you going organic or chemical? Pre-loaded soil or ?
Lots of options here, so figure out what direction you want to head and someone can help you...whether it be organic or chemical.
Also pick up some neem and plan your pest prophylaxis now. Set up a spray schedule (once a week or so) and stick with it. If done right, you shouldn't have any issues with pests or PM in flower (conditions granting of course).
That should get you going long enough to have some idle time while your new garden begins to flourish. Maybe you could use some of that time to do some reading ...
http://www.amazon.com/s/ref=nb_sb_noss_1?url=search-alias=aps&field-keywords=Cannabis+grow+book