a few things to consider.
that's a socket 775 board, so yes, that will be all but impossible to find new.
eventually you are going to have to replace it with a more modern gen one, and that'll force the upgrade of the RAM, CPU and PSU.
so serious thought is needed on whenever you go down that path now or go cheap with just a mobo and gamble the system hangs together for a few more years.
and even if you can just replace the mobo, XP will consider there has been too much hardware changes and will want to be reinstalled.