HP Business Desktop d500 Series Service & Reference Manual page 233

The system will automatically operate in single channel mode or a higher-performing dual
channel mode, depending on how the DIMMs are installed.
In single channel mode, the maximum operational speed is determined by the slowest DIMM
in the system. For example, if the system is populated with a DIMM that is 266 MHz and a
second DIMM that is 333 MHz, the system will run at the slower of the two speeds.
In dual channel mode, the DIMM pairs must be identically matched. DIMMs in the XMM1
and XMM3 black sockets must be identical; DIMMs in the XMM2 and XMM4 blue sockets
must also be identical. Therefore, if you have one preinstalled DIMM in socket XMM1 and
are adding a second DIMM, it is recommended that you install an identical DIMM into the
XMM3 socket.
Service Reference Guide, d500 336492-003 F–3