josh@nsgrounds.com, apologies for the delay in getting back to you.
That's a pretty major rebuild. Normally, I replace one component at a time and see what's happening. If everything's ok, then I move on to the next step.
There are probably too many permutations to work things out in your case.
The problem with XP is that it'll stop working if you replace too many things at one time because Microsoft has a sort-of "three strikes and you're out" policy. When you change the motherboard alone you're entering difficult territory because that changes too many things for XP to accept.
Assuming everything is cabled correctly, a fresh ubuntu install should be OK. But DON'T do it yet. I'd suggest that you try a few other Linux live-CDs on your system to see if you can get to a desktop without installing.
If you can get to a desktop, then you can pull off the important stuff from your XP install to a USB memory stick and keep it safe.
You could try Knoppix, PCLinuxOS or Puppy Linux to do this. Burn the images on another PC if you have one or use a friend's box if you haven't and then try them in your own machine.
Let me know how you get on and I promise I'll check this thread in the next few days so's I can reply.
If you can get any sort of a desktop - but DON'T install anything yet - you should be OK and then you can reinstall XP.
The worst case scenario - unfortunately - is that the initial partitioning before you installed ubuntu has done bad stuff to the XP partition...but I think you've probably just upgraded the machine more than Windows likes.