Memory Misreported

I have a system that was running a Tyan S2927-E with 8 GB memory, running Vista SP 2. Upon booting up, the Bios showed all 8 GB of memory and Vista reported 8 GB. I recently changed the motherboard to a Tyan S2932-E with no other changes. The bios still reports 8 GB. Vista and all related software now shows 4 GB. My son has the same configuration, Vista, 8 GB, Tyan S2932-E and his system also reports 4 GB. The change from the S2927-E to S2932-E changed the memory reported by Vista. There were no other changes. Anyone have any ideas. At this point, Tyan is pointing me to Microsoft, so I am in the middle of a finger pointing situation.

Thanks to anyone who might have some idea about this.