Assuming that you own a full retail license, you can upgrade any hardware.
However, replacing the motherboard will likly cause two additional problems:
(1) blue screen of death, (2) need to reactivate XP.
The first problem can usually be fixed by performing a "repair" installation
of XP to force it to load the right HAL for the hardware. However, that
will erase all XP updates. So, you will need to use the XP update feature
several times to get back to where you whwere. It will help if you have
SP-1 on CDROM. If all else fails, do a clean install. Obviously, backup
everthing you value before doing any of this.
The second is almost trivial in comparision. First, try the on-line
activation. That may actually work. If not, it should provide an 800
number to call. Within 5 minutes you should be reactivated.