WOW!!
My el-cheapo motherboard read the XMP from the SPD, but would not set it to PC2400... Now, with the correct memory settings, 1606cb on Cinebench R15! I am dumbfolded!
Current settings are x50 core multiplier and x44 cache multiplier. I did a 1h stress test and it went ok. Tonight I'll try running for 6h.