To have good airflow a 80mm in front blowing in and the PSU flan blowing out the back. For better air flow a 90mm in the front and a 60mm under the PSU blowing out+the PSU fan blowing out. For great airflow a 120mm fan in the front and a 80mm fan out the back+the PSU fan blowing out the back. The last is the conf I have and it works realy nice. Some people like to have more air flow in the case than exiting the case. "Positive pressure" its said to help keep the dust down inside your case. But I clean all those dust bunnie things out once a month anyway with my favorite computer tool.. Can-O-Air.
With "negative pressure" more air out than in, I think you get a better cooling effect. Anyone else?