The PowerMac cases are all basically the same case from the Blue & White G3 all the way up to the last MDD G4s, Apple just changed the fans and brackets from model to model.

The MDDs ran two G4 chips which also run pretty warm. The airflow in the dual MDD was definitely important since running with the case open would cause them to overheat pretty swiftly. As to whether its good compared to a P4, I don't really know. But I imagine dual 1.43GHz G4s would produce at least as much as a single P4.

If the air won't do it, you could always look into water.....