When installing DIMMs, consider the following DIMM limitations:
DIMMs are available in two speeds: 1066 MHz and 1333 MHz.
DIMM speed rules are as follows:
Three of the same kind of DIMMs per channel = 800 MHz
Two of the same kind of DIMMs per channel = 1333 MHz (for single-rank and dual-rank DIMMs); = 800 MHz (for quad-rank DIMMs)
One of the same kind of DIMMs per channel = 1333 MHz (if using 1333 MHz DIMMs)
One of the same kind of DIMMs per channel = 1066 MHz (if using 1066 MHz DIMMs)
The system operates all memory only as fast as the slowest DIMM configuration.