I haven't used Vista, and I've never had to reactivate XP (on multiple machines) despite changing a few things in all of them.
As for the BIOS and MAC address, these days that would make it motherboard-specific. I think it's more than that, and I remember reading something in the past where there is a threshold of how much can be changed before reactivation is triggered. It wasn't clear to me at the time. I'm not entirely sure.
My brother runs his own PC tech support business, and had heard that WAS the case, so he was naturally a bit nervous about what the future was bringing to him. (In the past, if he spent x hours reimaging a PC, with Vista it might be x hours + y hours now. Not including phone support with Microsoft, which has turned him, on occasion into nearly a raving lunatic)
I have since heard that Microsoft changed it prior to the general release of Vista, so changing the motherboard would not require the hoops, but...I have not heard any confirmation on that.