Just curios why your thread title is pcspecialist build for £1000 and then you post a link to argos.
Anyways , regardless of that i can tell you that pc specialist are very good at what they do , they even advertise if you find your build cheaper tell us.
I have a very expensive games rig from them and a cheaper every day rig that i use to try thing on before i put them on my main rig.
First look at the required specs for games you intend to play and then go back to the build my pc column on their website , at the end of the process their system will check for mismatched parts or overkill such as too big a power unit.
If you do go with them take out the breakdown insurance , i did and was glad i did because 1 year later some of my ram died.
Also if you have a side fan it will not be connected when you get the pc because the wire would snag on their special transit packing.