Sounds like the first thing you need is a more efficient CPU fan / heatsink.
If the top of the heatsink (near the fan is hot to the touch, you don't have enough fins or fan.
If the base of the heatsink (near the CPU) is not as hot as the CPU, you have poor thermal transfer.
I use power supplies with dual fans for any high performance system. This helps move the hot air from the CPU out of the case.
For case fans, while I opt for positive case pressure, I use fans with a thermal sensor. They speed up as the sensor gets hotter. That way, I don't need to remember to turn them on and they are reasonably quiet when the PC is idling.
You do want to nearly match airflow in to airflow out. More than a 10% difference in CFM in versus out can create a stagnant case.
Since you are doing video rendering, do you have a slot fan next to your video card?
Chas