opensuse - Missing memory (BIOS detects 8 x 2GBs but says only 12 GBs installable)

OS: OpenSuse 12.3 (x64)

Problem: Unable to use all detected memory

I recently bought some more RAM for my workstation (2Rx4). The motherboard has 8 slots, and while BIOS recognizes that all 8 have a 2GB stick of RAM, the "Total Installable" is only 12GBs.

dmidecode tells me the maximum supported RAM is 32GBs, so I suspect this may be a rank issue, although I'm not super familiar with memory.

Update

To be explicit - does anyone have any ideas why I am unable to use all the detected memory?

I had 8GBs before in a 4x2GB configuration.

RAM details

2GB 240p PC2-5300 CL5 36c 128x4 DDR2-667 2Rx4 1.8V ECC FBDIMM

All 8 slots are registered as active (see dmidecode output) and the BIOS detects all 8 sticks

Motherboard details

Base Board Information
    Manufacturer: Dell Inc.
    Product Name: 0GU083
    Version: A00
    Serial Number: ..CN1374073H00UP.

Available memory (BIOS also says 12GB installed memory, but lists 8 2GB sticks)

             total       used       free     shared    buffers     cached
Mem:         12039      11888        151          0        549      10825
-/+ buffers/cache:        512      11527
Swap:         1992        573       1419

Full dmidecode output here

UPDATE 2
memtest x86 only detects 12GBs of memory too;