You will need to re-install the operating system again. Anytime I have changed motherboards where I tried using the old hard drive I was never entirely successful. I don't even try anymore, it's just too much of a hassle.
When you re-install XP2 you may find out that your version of XP was an OEM version that isn't transferable to the new system. As long as your copy of XP is a full version you should be okay re-installing it. Most certainly you will have to re-register XP. The video card and network card being different will automatically make you have to re-register your copy of XP when you install it in the new machine.
I have seen where some people have said that they told Microsoft that the motherboard was fried and Microsoft gave them a new serial number to use to register XP on the new machine, but I have not done that myself.