I'm still working on my quest for a small reliable server.
I was just reading Richard Elling's blog entry on RAS and the X4100/X4200 servers. You should read this - blindingly obvious design features like not putting heat generators like disks in the airflow path for the CPUs. But he also says that most thin servers don't need more than 2 disks. Perhaps this is why I'm having so much trouble finding a server to fit my requirements!
Oh well, having exhausted Sun's catalog, I'm now looking at the likes of the HP DL385 or the Dell PE2850. Both of these are listed in the HCL, which is pretty much essential as I would naturally be running Solaris on the machine.