Need Help upgrading

I’m looking to upgrade and want to spend around £300 to £400. I want a mother board which will allow me upgrade over time add better graphic card more ram etc

I’m pretty new to PC building and think that changing my motherboard (asus??), 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)