The "vacuum" a fan's vacuum is so small on a pc fan it is futile. Have fans blow all in making a pressure build up of cold air for heat to transfer too.
I'm with Grouter on this one, single 120mm exhuast fan seems to work best.
http://icrontic.com/articles/pc_airflow_heat_cooling_guide
http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/cooling_guide.pdf
http://www.endpcnoise.com/cgi-bin/e/computercooling.html