X51 R3, general upgrade

Hi, I browsed around for this topic but most posts are a bit dated, so I'm asking for your advice about a general upgrade for my X51 R3. It works fine but I'm not able to play AC origins with full graphics and the SSD is almost at capacity. Any ideas, which upgrades worked for you (and fit properly in the chassis)? Thank you.

NVIDIA GeForce GTX 960 with 2GB GDDR5

256GB M.2 SSD + 2TB 7,200RPM

16GB (2x8GB) DDR4 2133MHz SDRAM Memory

Intel i7-6700K Processor (4-Cores, 4.0GHz)

Alienware X51 R3 Chassis Liquid Cooled without Optical Drive

Alienware 330W AC Adapter