As hamluis has said, with the original DDR2, the clock speed is half the DDR transfer rate, as DDR2 double-clocks data transfers (where the 2 comes from). So DDR2-800 is clocked at 400MHz.
For the Phenom, an AMD cpu with on-chip memory controllers runs the memory clock by dividing frequency down from the CPU clock, so the ratio is fixed and determined by the CPU. DDR3 also double-clocks like DDR2 (other enhancements earn the "3" status) so the DDR3-1333 rate that this CPU runs the memory means the clock is 667MHz (3GHz CPU clock/4.5).
Having DDR3-1600 RAM fitted would allow for potential overclocking. Your CPU is divider locked, so overclocking can only be done by increasing clock speed, which will overclock both CPU and memory.
Edited by Platypus, 21 July 2010 - 06:20 PM.