Q: Why can't my pair of DDR400 memory modules operate at 400MHz FSB on DIMMs 2 & 3?(KV8-MAX3)
A: According to the AMD® AMD Athlon™ 64 Processor Data Sheet on page 16m table 3 regarding unbuffered DIMM support, the AMD Athlon 64 Processor does not support DDR400 memory operation at 400MHz FSB on DIMMs 2 & 3.
http://www.amd.com/us-en/assets/content_type/DownloadableAssets/dwamd_24659.pdf