"The ASRock Fatal1ty 990FX Professional supports memory of only 1GB or 2GB sticks, above 1600MHz, I found that surprising. I thought 4GB sticks, were the new standard. "
How do u figure? It works fine.
Are u mixing the RAM?
Number of Memory Slots - 4×240pin
Memory Standard - DDR3 2100(OC) / 1866 / 1600 / 1333 / 1066 / 800
Maximum Memory Supported - 32GB