try putting a pci slot fan in the top pci slot.  heat rises anyway and one of these will blow a lot more of it out of the inside.

found one on ebay:

I want to add one to my mdd.  I read an article on xlr8yourmac that one of these lowered and mdd from 59-60c like mine down to 54-56.
Mac mini C2D 1.83GHz - 2GB RAM - 200GB 7200rpm HD
Sawtooth w/G4 1GHz - 1GB RAM - SIIG SATA w/2x1TB HD
B&W w/G4 600MHz - 1GB RAM - 2x80GB HD
Sawtooth G4 400MHz - 512MB RAM - 40GB HD
Dell Ultrasharp 2007WFP 20" Widescreen LCD
2.94TB Total Storage