by default 1333MHz ram running 667MHz times by 2, 667MHzx2=1333MHz
that is means by DDR (double data rate).."
DRAM is single rate RAM. DDR1 is double data rate ram and has the ability to read or write two words per cycle. DDR2 SDRam can read or write four words per cycle. DDR3 ram doubles the previous and can write or read eight words per clock cycle.

if u have 800MHz in cpu-z means ur ram running 1600MHz natively