The memory speed in a server will depend on several factors: CPU, number of memory ranks installed and voltage.

Your CPU may only be able to run memory up to a certain speed, regardless of the memory you have installed. For example, you may have 1600Mhz memory installed, but the CPU is only capable of running memory at 1333Mhz.

Memory ranks can affect speed the more modules you have installed. For example, if you have a system that runs memory in triple channel and install one set of 3 modules, it may run at 1333Mhz if they are single or dual ranked. But once a second set of modules is installed, the speed is reduced to 1066MHz. If the first set was quad ranked, it may only run at 1066Mhz and down clock to 800Mhz with a second set. Quad ranked modules would be used because they are usually higher capacity. Some systems are compatible with load reduces DIMMs (LRDIMMs). These modules allow for higher capacity modules at higher speeds.

If you install low voltage DIMMs, They may run at a slower speed than the same number of standard voltage DIMMs. For example, you may be able to install two set of 1.5V memory at 1333Mhz. But with 1.35V memory, the modules would run at 1066Mhz.

Please refer to your system or motherboard manual for specific memory configurations.