DDR2 800 memory on P5B Deluxe

Hi,
I have a P5B Deluxe motherboard on which I have installed 2 sticks of 1 GB ddr2 667Mhz RAM about four years ago.Now I would like to increase its memory. The only memory modules available now are ddr2 800Mhz.
My doubts are - 1) can I install two sticks of 2GB ddr2 800 memory keeping the older ddr2 667 memory on board. What would happen if I do so? Would the 800Mhz memory be automatically clocked down to 667Mhz.
2) Would it affect the quality of the 800Mhz modules in any way.
3) Later if I add 2 more 800Mhz modules would all the modules automatically clock back to 800Mhz.
4) For performance sake would it be better to remove the 667Mhz modules and install only the 800Mhz modules.

Plz Reply. Would be great help. THanx in advance