Motherboard supports DDR3 1333(OC)/1066/800 non-ECC, un-buffered memory.
Core 2 Duo CPU supports DDR3-1066.
New RAM is DDR3-1600.
The motherboard was able to support DDR3-1333 RAM and run it as DDR3-1066 for the CPU.
The motherboard does not support DDR3-1600.
Eh, that's what I thought, only I was somewhat confused by when it is possible to run DDR at lowered frequency.
Don't put old and new sticks together and check. Sometimes new stick may have manufacturing defects.
I did not, I was testing them one by one.