1) System changes are recorded by Vista on a scale of 1 to 10, therefor you can perform multiple "small" upgrades such as graphics cards, hard disks, optical drives etc without having to re-activate. You can find in depth information on this via Google.
2) Think about it, if a motherboard in a shop bought computer failed and had to be replaced, do you seriously think you or the manufacturer would have to pay for a new copy of Vista? Of course not, that said you may have to call Microsoft (sort of like with XP activation), and as long as you give them a genuine reason for the change (such as hardware failure) they will either "reset" your copy of Vista to allow re-activation or issue you a new product key which would then be assigned to your new motherboard and / or other hardware changes.
So basically you can change your motherboard and / or other hardware without having to purchase a new Vista OEM license, BUT if you do it too often they may eventually refuse to help you and request you buy a new license as technically speaking it's no longer the same system. So unless you frequently change your motherboard, like more than once a year or something, chances are you'll be fine with the OEM version.
