There is a great explanation here :
http://www.technibble.com/windows-xp-activation-explained/
To summarize, there are 10 votes by the hardware devices, when you boot, that, "Yes, I'm the same". If you ghost one disk to another disk, then one vote is used up and your OS voting is 9/10 saying "yes, I'm the same." If you change the NIC that counts as 3 votes. So while you can ghost your hard drive to another disk, if you do some other major change (like the NIC, or changing the CPU + motherboard + audio), then you will get to 6 votes and you will have to reactivate.
As for OEM "motherboard locked" versions of Windows, I don't know if these rules apply for those versions of windows.
Due to the way voting works, I don't think migrating to a new disk 3 times uses up 3 votes; it would only use 1 vote because the voting just says, "gosh this isn't the same disk I was activated with" and it doesn't care about subsequent changes in your disk drive.