I don't know to what extent you can change pre-built rigs but ill tell you what i'd do (if it wasn't a pre-built)
Only get 8GB of RAM, 16GB is too much for Gaming, 8GB will be fine.
Get the 8320 instead of the 8350, cheaper and basically the same chip.
Get a different cooler, either the H80i or Tundra (240mm version, while its £85)
A 750w/850W PSU for Xfire later on.
Switch the case for something with better airflow, maybe a Ronin (if they ever re-appear)