Hi guy's I have about £400 to spend for Christmas, and wondering what i could possibly upgrade for that.

here are my current specs:
ntel i7-6800K 3.40GHz (3.8GHz Turbo) Socket 6x Core Processor
PALIT NVIDIA GeForce GTX 1080 Ti 11GB Founders Edition
Corsair CS650M CS Modular 80 Plus GOLD-RATE 650W 12V Power Supply
Gigabyte GA-X99 Ultra Gaming RGB Motherboard
Corsair Vengeance LPX 16GB (2x8GB) 3000MHz DDR4 Memory- Black
Corsair Hydro Series H75 Performance Liquid CPU Cooler
NZXT Noctis 450 Matte Black Full Tower Case
Windows 10 Home 32/64 bit Operating System
250GB Samsung EVO 850 SSD - MZ-75E250B/EU
2TB Seagate Barracuda SATA III Hard Drive