Hi-
You are running right at the border of CPU freezes with that temperature.
With 4 hard drives, and the heat from the CPUs, you are really taxing the cooling ability of your machine.
Do you use CHUD tools for Nap mode in the processors? CHUD will help with CPU temps, normally, but under load, and during heavy hard drive activity, the heat from the hard drives will combine with the processor heat to keep temps high.
You might consider getting an external firewire housing or two, and moving the drives from the ATA/66 bus to the external housings.
The large case fan, 120 x 120 x 38mm may be slowing down, or dirty, or may just need to be replaced with one of higher CFM. The 3,000RPM
Scythe Ultra Kaze will boost the airflow inside the case over the OEM fans 90CFM ability.
Another possible aid would be a 32CFM (or more) PCI slot fan to exhaust some of that hot air from the case.
At a minimum, make sure all intake and exhaust vents in the case are free from dust and lint. The same is to be said for the CPU heatsink, all components, and the interior of the PSU.
Compressed air will help the process.