Asus P8B WS - Manual (Page 31)

Recommended memory configurations
2.2.4 System memory
The motherboard comes with four Double Data Rate 3 (DDR3) Dual Inline Memory
Modules (DIMM) slots.
A DDR3 module is notched differently from a DDR or DDR2 module. DO NOT
install a DDR or DDR2 memory module to the DDR3 slot.
ASUS P8B WS 2-5