Be aware that only an eMachines motherboard will work with that version of Windows XP recovery they sent with the machine. So you will need to add a valid Windows XP OEM.
Also, the hard drive won't work with the new motherboard until you install Winodws in the repair mode.
The replacement motherboard is available from eMachines for $159.95 to $211.95, but it will likely be no more reliable than the one you now have. eMachines motherboards have the highest failure rate of all computerdom.
You can get Windows XP Home OEM for $64 to $89 at discount online sites, or $99 at Wal-Mart