pc upgrade. too many options

I'm looking to upgrade and want to spend around £300 to £400. I want a future proof mother board which will allow me upgrade over time add better graphic card more ram better cpu etc
I'm pretty new to PC building and think that changing my motherboard, CPU, ram and hard drive would be best, but I'm not sure. My goal is to have something which will play most new games on high setting.
Any ideas for upgrade/build would be a great help thanks.

This is my PC currently-
CPU intel core 2 X6800 @2.93Ghz
Cooler Freezer 7 pro
Ram 4Gb ddr 2
Motherboard Asus P5W DH Deluxe
Graphics Geforce GTX 550 Ti
HDD Samsung HD501LJ 500GB
Power ZS series 750w
Case Thermaltake Armor VA8000BWS
Optical pioneer dvd-rw 111d (X 2)

Corsair CMZ8GX3M2A1600C9 8GB 1600MHz CL9 DDR3 Vengeance Memory Two Module Kit with i5 2500k processor and asus motherboard z77 and artic cooler 13 cpu fan then when you get more cash down the line you can get another pack of memory thats what i did then got 3tb hard drive and asus hd 7950 oc v2 cooler i also got a nice little 3d lg monitor from argos hope this helps