I had suspected that what drove you to where you are at was a dead motherboard.
The easy question first. Because most of your system has changed, it will trigger an activation event. How much of a pain this is depends on your version of XP, and how many times you have had activation events.
If you have the retail or upgrade version, you should be fine. If you have an OEM version, Microsoft may require you to buy a new license. This will depend on how long it has been since your last activation, and how much hardware you bring over from the old system.
Now, as to the RAID, if you read most manuals for setting up a RAID array, they warn you that you may loose data that is on your drives. How high your risk is depends on how compatible the 2 different RAID controllers are, and "IF" the new controller recognizes your existing RAID configuration.
The best suggestion I have for backing up your existing files would be to install a PCI RAID card that uses the same brand and model controller as was on your old motherboard.
Alternately, you can set up the system and enter the RAID configuration program. If the software is written well, it should let you know there is an existing RAID configuration and if it can use the existing RAID or not.
If the RAID configuration software does not recognize your existing RAID or states that it cannot use the existing configuration, then you know you need to find a way to back up your files.
Remember, all you want to do at this time is verify that the new RAID controller recognizes your RAID configuration. If so, you should be able to proceed with caution.
If it does not recognize your RAID configuration, "DO NOT" allow the software to configure your array.
It would not hurt to contact tech support at both the motherboard and RAID controller manufacture. There might even be a FAQ on recovering an existing RAID configuration or a list of compatible configurations.
And yes, I am erring on the side of caution.
Chas