If your motherboard only supports PC2100, thus putting PC3200 in it, your board will Underclock it at PC2100 speeds. But if you only want to use PC2700 so you can OC more. That will work, because the RAM can handle faster speeds then PC2100.
Dont forget Gigabyte's arent good for OC. So dont expect to much. You can use any kind of processor to hit 2800 speeds. If your using a 2600, you will have to OC less. Opposed to using a 2200. Just dont forget that heat will be a big issue. The more you OC, the more heat you will inherit. So make sure you have some type of temperature monitor.
Finished With TF.