I should have prefaced my advice. I don't have any direct experience with a lot of this, so you want to double check what I say. I've read a lot about it though. The best info about this I read on Al B. Fuct's monster thread about how to get a harvest every two weeks. My grow is extremely ghetto with just a box fan venting into my attic. But my grow is in a closet and it's a big enough space where I have to do very little venting on top of that.
According to what I read, you want to have a separate fan drawing the air into the cooltube before the light, before the air gets heated. Then you want to dump that heated air outside of your cabinet. Ideally you want two separate air systems, one for the cooltube (that hot air never enters the cabinet) and one for the cabinet itself.
The reason for having two seperate systems, is that the fan for the cooltube, you want running at max speed, all the time, keeping the cooltube as cool as possible, so you can get the light as close to you plants as possible. However, the speed of the intake fan for the main cabinet ideally would be controlled by the temp of the air in the cabinet. You want those temps to be as stable as you can get 'em, especially to keep humidity constant (as air cools it dumps moisture.) Assuming that the temps of the intake air fluctuate with the seasons, the only way you can do this is to have a fan that has it's speed controlled by a temperature sensor inside the cabinet.
You could get something like
this to control your fan, but I have no experience with this product specifically. It seems that it would be able to turn the fan on and off with your preset temperature. Sadly, it won't act like a rheostat, controlling the
speed of the fan, which would be more ideal. But it's pretty damn cheap solution for what it does.
I suppose you could have the heated cooltube's air dump into the same exhaust. But for the price of an extra hole in your cabinet, I'd keep them separate just so you have more control and you only have to carbon filter a much smaller amount of air to control scents. If an intake fan wasn't strong enough to do the job for you on either system, you could always add a second fan on the exhaust side of things (but get it on the rheostat, same as the intake for the cabinet.)
Of course, all of this is assuming that you have intake air that is cool enough to keep your cabinet the right temperature. If your intake air is just too damn hot, then you're going to need some A/C.
Oh, one more thing, you have your ballast outside the cabinet, right?