Assuming you run the RAM with a 1:1 ration with the FSB...the DDR2-667 will allow you to OC the CPU to 2.33GHz without OCing the RAM. DDR2-800 will allow you to get to 2.8GHz without OCing the RAM.
If you want to reach 3.0GHz, you'll be MUCH better off with DDR2-800...you'll have to push the FSB to 429MHz, which will put the RAM at 858MHz (DDR)...most DDR2-800 will run at this speed with a little extra voltage. But you'll have very little chance of getting there with DDR2-667.