Generally, RAMs of different speeds MAY BE used together. If it
is possible, your motherboard will automatically fall back on the
slowest speed available.
You will need to find out your motherboard's model and the type of
RAM it supports before using RAMs of different speeds.
RAMs of different sizes can be used together, provided that they
are of the same type.
If you have your motherboard's manual with you, read through it.
Otherwise, download CPU-Z. It will
tell you your hardware configuration.