It turns out we were misinformed by an incorrect packing slip. I apologize for the red herring. These are actually Tyan GT20 boxes with Tyan K8SSA (S3870) motherboards. They have 6 DIMM slots and use a ServerWorks BCM5785 (HT-1000) chipset. The config of DIMM slots is kind of weird: there are four nominally supporting the first processor and two supporting the second, though the processors can read from each other's slots. As for mixing RAM, we did try 4x1GB of identical DIMM's, and got the same problems with i386 CentOS 4.4 . x86_64 is fine. Dan