The HP EliteDesk 800 G1 TWR uses four DDR3 DIMM slots for up to 32GB

The tower variant of the G1 gets proper desktop memory. Four 240-pin DDR3 DIMM slots running at 1600 MHz, non-ECC, maxing at 32GB. The four-slot layout means you can populate it gradually — start with 8GB, add another stick later, and still have room for more.