It is recommended the RAM specs be the same (voltage, CAS latency, ECC/Non-ECC, etc.). Mismatched RAM can cause problems such as random shutdowns and other problems.
"NON-ECC/Non-parity — Most desktop and laptop computers take NON-ECC or Non-parity memory.
ECC/Parity — ECC or parity modules look for errors in data and are most often found in servers and other mission-critical applications used by large networks and businesses.
Unbuffered — Most PCs and workstations use unbuffered memory which is faster than registered memory.
Registered/Buffered — Registered or buffered modules delay all information transferred to the module by one clock cycle. This type of memory is primarily used in servers.
Fully buffered — Designed for next-generation servers, features an advanced memory buffer.
CL — CAS (column address strobe) latency, which is the number of clock cycles it takes before data starts to flow after a command is received. Lower CL is faster. Modules with different CL can be mixed on a system, but the system will only run at the highest (slowest) CL.
Component configuration — (For example: 64Meg x 64) Indicates the size of the memory chip components on the module.
Voltage — For example 2.6V. Indicates the power used by the module. The lower the better.
Check the glossary and FAQs for more detailed definitions. "
The above was from crucial.com