An intake fan on the front of the cast and an exhaust fan in the back (*not* the fan on the back of the power supply) is standard operating procedure these days. You have to have both fans. Just having an exhaust fan creates a slight vacuum that will raise temperatures, and just having and intake fan is useless, because you want to move hot air out, instead of just adding more air.
I also suspect that the heatsink was not properly installed (flush with the CPU and glazed carefully with compound).