So I encountered an interesting problem this week: too many ranks. Apparently Opterons can only handle 4 ranks of memory at 400MHz. We loaded a certain machine with 4 dual-rank DIMMs per CPU (=8 ranks/CPU) and it was woefully unstable. Opterons can handle 8 ranks if they run at 333MHz, so I downclocked the memory in the BIOS.
Just something to watch out for the next time you try to build a 1U with two dual-core Opteron 270s and 8 2GB DIMMs. :)
This machine pulls 2.9A under load!