The E8400 has a 1333 FBS (333x4) and the RAM would be 667 (333x2). With a 1:1 ratio.
If you are goung to overclock IE 1600 FBS (400X4) you would need DDR2 800 RAM (400x2).
The E8400 can overclock over 400 FBS, So if you are going to overclock get fast RAM, the faster the RAM the higher you can overclock.
The P5K Pro is a good board. ASUS makes good boards and if it has all the options you want then it is the right board for you.