Novatech are good.
Alienware are greatly overpriced.
Dell don't make gaming machines, whatever their claims may be.
Have heard bad things about Cyberpower.
Mesh are bad. Very bad.
Have never heard of UK Gaming Systems.
sli/Crossfire imo a waste of money, one good card, possibly a dual GPU, a better bet.
You have an ample budget for a very respectable gaming machine.
i7 is the way to go.
6Gb memory is the sweet spot.
Do you already have peripherals? Monitor, mouse, keyboard, speakers?
I would assume so.
Take a look at this:
Gaming Machine
That will give you an idea what OCUK provide.
You can order a machine from them to your own design. With your budget you could use an Nvidia 295 series card
Link.
The ideal would be to use an SSD hard drive to take the operating system
Link with a 1Tb SATA hard disk to take your games and storage.
Blisteringly fast.
Novatech also offer good gaming machines.
I don't honestly know of any other suppliers to recommend cos I always build my own.
The above links are just examples, suggestions.
Note that Nvidia high end cards are a little thin on the ground at this moment in time but suppliers, hopefully, will get fresh stock soon.
Also budget for Win 7, Home Premium costs on average about eighty quid.
Oh, and welcome to the forum btw.