both have a range of numbers that they run at. Just as an example since I dont know the exact numbers, if ddr2 can run anywhere between 500-1000 mhz then ddr3 can run at 1000-1500 mhz though the exact number that any individual memory stick runs at is fixed somewhere in that range. Overclocking might be able to change the mhz rating. The ddr2 with the highest mhz rating is about the same mhz as the slowest ddr3. I think it might be 1200 or about there were the two meet.