With respect to booting with a conflict in the system memory map: the Windows behavior is the right one, IMHO. An operating system shouldn't just ignore self-contradictory system configuration information and try to continue anyway. Who's to say that the resulting behavior is the right one? You're in undefined behavior territory. Failing fast when encountering invalid state, as Windows does, leads to more robustness in the end: fail-fast gets problems fixed, not swept under the rug.
Fail-fast only has a hope of getting the problem fixed if it occurs with popular software within a few months of product launch. In the case of using a motherboard with DIMMs that didn't exist until the MB was obsolete, you're never going to get the vendor to fix it, unless you have a very expensive long-term supply and support contract of the sort that doesn't exist for consumer parts.
