For HID a small vent is usually never enough, you're gonna want to invest in some good ventilation fans. Ventilation is not something that you should cheap out on by any means, it's very important. In fact, with proper ventilation you won't need to blow the AC all the time. Since you have 600 watt lights I'm guessing your hoods have 4-6 inch duct ports right? If you don't have a nice hood with glass underneath and some ventilation holes you should definitely invest in some. You can go on Amazon and get some really well priced fans, same stuff as the hydro stores but half the price! Craigslist might also be worth looking into, but you'll definitely want some ventilation.
http://www.amazon.com/Output-In-line-Exhaust-Scrubber-Filter/dp/B004OH0DIE/ref=sr_1_10?rps=1&ie=UTF8&qid=1362972110&sr=8-10&keywords=inline+fan
8 inch 720 CFM fan for $100 bucks, if you had one of these bad boys for your rooms exhaust you'd be all set. You say you have a small vent that's venting air outdoors, but what is actually likely happening is that air is coming in from outside instead of going out like you think.. however this is actually a good thing. If you put another small vent at the bottom of your box you'll have a nice passive intake going for the box itself. Then you buy yourself the fan I listed above or this 6 inch 440 cfm fan
http://www.amazon.com/dp/B006OQTY6U/?tag=chrome-purchreview-20. You'll take the new fan that you bought and connect it to a hole in the top of your box (because heat rises). The reason you want to do these two things is because this will circulate the air in your box itself. Fresh air will be sucked in from your 2 passive intake holes and the hot used air will be blown out with your 440-720CFM fan and your plants will be loving you.
Now for cooling the lights. You'll want to make an intake hole on the bottom of your box and connect it to an outside source of air if possible. Then get one of these bad boys.
http://www.amazon.com/dp/B008N4QIZG/?tag=chrome-purchreview-20
These fans aren't super powerful fans that cool the actual bulbs themselves, but rather it just helps air flow into your light. So since you have 2 600 watt lights I'd do something like this.
Intake>small booster fan>ducting>hood>ducting> booster fan> ducting> hood>ducting>720CFM fan>exhaust
If you do this you'll be able to touch the glass on your hoods which means your lights aren't adding as much heat to your box as they otherwise would. Putting the ballast outside the box also helps.
Let me know if you have any other questsions, I'm no expert but I'd be happy to help where I can my man.
