Using 16GB memory modules in 8GB slots
This seems to be possible often with I'M Intelligent Memory | Beyond Limits, a company that stacks memory units beyond the specifications. A compatibility list is here: compatibilitylist.pdf, and you can buy them here in Europe: Memphis Electronic AG @ Suchergebnis auf Amazon.de (a bit pricey I'd say, may be cheaper to buy a motherboard with more memory slots).