|You don't necessarily need to know what motherboard is in it, although that information is usually printed on the motherboard somewhere. It's probably an OEM boad from Asus or MSI or similar manufacturer. Unfortunately a manufacturer won't have info on its OEM boards but sometimes you can match something similar.|
The best place to start would be the emachine download center:
and see if you can find anything there. Or try googling the emachine model number along with something like 'cpu upgrade' and see what you get.