So, I'm building a new PC as well as retro-fitting an older one for the kids. I've got fans flying everywhere, and my question is - in general, do I want the fans to suck out the hot air in the case or blow into it?

It seems the power supply sucks air into it and out the back of the chassis, and the CPU fan blows air off the CPU to disperse the heat. What about the 80mm fan that's located on the door of the chassis? What about the 120mm in the back of the chassis?

For optimum cooling. Fans should both blow and suck!

Rig one on the front as an intake or output, and one or more on the back to do the opposite. You want air flow. Maximum Airflow even.

There are different camps on the ratio of blow/suck. (can't think of a better way to put it)

Some people feel that creating positive pressure, where there are more intake fans than exhaust fans is better, since that creates more possible air movement since the air has to travel more before getting exhausted, absorbing more heat. Others feel that the opposite should occur, since they feel that more direct airflow over the components is better.

Not sure which approach is best, if there is much difference at all.

She's gone from suck to blow!

The key is to suck new air in from the front of the case and blow it out the back of the case (like the power supply fan is doing). You want movement through the case. Ideally put a fan in each spot your case has; my Antec has two in front and two in the back.

In most cases, you don't need to get too worried about all the other details being talked about. Usually a fan or two on the front and a fan or two on the back is fine. It's the overclockers (who generate more heat, and get a more reliable overclocking from getting rid of that heat) who get truly picky about pressure differentials and all that stuff.

Also look for variable-speed fans and a power supply that can drive them. It can help keep the noise down by driving the fan speed based on case temp.