Your motherboard isn't running at 800mhz, your RAM is.
A typical intel motherboard will have the CPU running at 1066, 1333 or 1600. The ram running at 533, 677, or 800. Other things like Hard drive, video card, etc all have their own mhz speed as well.
What you should do is check the CPU speed of the motherboard.
If the socket is correct, and the bios has been updated to support it, you can insert a higher speed CPU and just have it work at a slower spped. (aka, if you put a 3ghz 1600mhz CPU in a 1066mhz socket it will work below the 3ghz mark.)
But again, check the motherboard's compatibility list.