You have to see if your motherboard with accept an 800 mhz procesor-check the manual. If it doesn't off hand, then check the MOBO manufacturers Web page to see if flashing your BIOS will make it compatible with 800 mhz processors(read the instructions Carefully if you do flash the BIOS CAUSE if things go wrong you could have some real problems). If your boards bios is not upgradable then you are SOL. You will have to buy a new MOBO to take advantage of a 800 MHZ processor.
You will also have to see whether your processor is a Slot or a socket version. If it is a socket version you should have no problems getting an 800 mhz chip for the MOBO(if the MOBO can use it- read above). The slot version is older technology and may not be available @ 800 mhz (I do not know for sure- I am sure some intel zeolot will have the answer for you, if you ask).
Another thing you should watch for. If your motherboard does accept an 800 mhz processor and the configuration(slot, socket) is not a problem. You should make sure that you buy the right type of Intel processor. Intel builds two types of 800 mhz processors 1.) a 100 mhz bus 2.) a 133 mhz bus. You will have to see which bus type your MOBO uses.
The 800EB(133) is better for non-overclocking systems because it has a higher bus. The 800E(100) is better for those that want to overclock there systems because they can up the bus frequency to 133 mhz if there MOBO is designed to 133 mhz tolerances. Obviously 800E(100) is the only choice if your MOBO only operates @ a bus frequency of 100 mhz.
As far as switching back that should not be a problem. As long as you haven't screwed anything up on the MOBO or the connections to its peripherals(i.e. HDD, Power, IDE cables etc.). Oh ya, make sure you ground yourself before touching any components in your case cause a little static electricity could mean the end of some of your components