I currently use a stick of 1GB DDR2 667mhz and two sticks of 512MB 667mhz RAM in my current system (2GB total).
If I were take all of these out and put 2 sticks of 1GB 800mhz in there instead would I be able to use any of the 667mhz RAM? If so, would it be worthwilfe or should I just sell the old 667mhz on Ebay? My motherboard supports both types of RAM
Definately. Even if the 800Mhz sticks were only running at 667MHz, the benefit of running in dual channel mode would be worth it. Other than that, you probably would not notice a big difference between 667MHz dual channel and 800MHz dual channel, especially if your bus speed is 1066MHz or 1333MHz, since the 1:1 FSB RAM ratio would have your memory running at 667MHz or slower anyway.