If the motherboard dies you can replace it if you have OEM. More info below.
What Exactly Can One Change With An OEM Operating System?
You shouldn't have a problem replacing any parts other then the motherboard, although if you replace too many of the computers parts you may need to reactivate. If you replace the motherboard you will need to reactivate (and likely reinstall), which (according to some) you can do. However there is also some OEM documentation stating that replacing the motherboard for reasons other then defect creates a new computer and the OEM license is no longer valid.
Many posts here say you can replace the motherboard, many others say you can't. Some of the confusion is because the OEM EULA doesn't specifically say replacing the motherboard creates a new computer, that language comes from a FAQ page about the OEM license (and a FAQ page is not part of the EULA). But Microsoft also has another OEM webpage saying you can replace the motherboard and still use the OEM license - causing much confusion on this topic.
Because Microsoft says you can replace the motherboard on one page and the OEM EULA doesn't say otherwise, you should be able to replace the motherboard. However, if your Windows install disk is from a computer manufacturer the disk might be locked to the motherboards BIOS which would prevent you from being able to install Windows on a motherboard not made by that computer manufacturer. Also, OEM copies are bound to the first computer installed on and are not transferable (can't be moved to a different computer).
OEM License Page:
OEM FAQ Page saying you can't replace the motherboard unless it's defective (8 questions down):
This page (about Vista) suggests you can replace the motherboard with OEM, it just needs to be reactivated:
Hello, this is almost exactly my situation - and I am not replacing my faulty motherboard with the same or a similar one. God forbid why on earth would I do that??!!. I have taken the opportunity to order the 1156 mobo with an i5 cpu. The new mobo will cause an issue with licencing so I am interested in finding out how you (JoeyBeanz) got on?