You will need to reactivate XP if you do a clean reinstall. If you keep all the hardware the same, including NIC, and you keep the same type and amount of RAM, you should be able to reinstall WinXP and re-activate on the internet. If you change too much hardware, you will not be able to reactivate by internet, but you will be able to reactivate by phone call to MS, which is a hassle but painless. Just tell them that you upgraded your mobo. You are not allowed to install XP on more than one machine, but you are allowed to change the hardware in that machine.
You can avoid the necessity of reactivating XP by doing a Repair install. This almost always works, but if you have a problem, you can always reformat C: and do a fresh install.
To do a Repair install, first copy the wpa activation file to a floppy or a partition other than C: so you can retrieve it if needed later. Copy C:\Windows\system32\wpa.dbl which contains the XP activation info. Usually after a Repair install, XP does not ask you to re-activate, but if it does ask you to reactivate, you can copy the original wpa.dbl file back into \system32 folder and that may reset XP to the activated state. This only works for a Repair install, not for a new install, and it only works if you did not make too many hardware changes, in which case you can call MS to reactivate.
After installing your new mobo, be sure to set the bios to boot from CD the very first time you boot. Boot from the XP CD. The first time XP asks if you want to Repair, say "no" but the 2nd time XP asks if you want a Repair installation, say "yes". XP will go through the whole installation and reinstall all the drivers for your new mobo, but will maintain all your XP settings. After installing XP, use your mobo driver CD to install drivers for your mobo which XP may not have installed or to replace generic XP drivers.
I have done this with complete success several times, but I did have one mobo change which required a clean install of XP for unclear reasons.
Good luck with your new mobo!