It is not necessary. The 800 mhz RAM will step down to match the lower 667 mhz RAM automatically. You will have to replace or remove the 667 mhz RAM to operate at 800 mhz, if your motherboard supports it.
Just one to note, it may be worthwile to take out the old memory and install new if you have the money. The speed difference between 667 and 800 is not alot but also look at latency timing for the chips. More often than not chips will work together but sometimes there are problems. The only way to find out is to try it then run a memory test. Look at the memory in my spec for a good buy.