Since you are in the UK, scan.co.uk is your best bet. The prices are the cheapest you are going to get (Its a Computer only shop) and the products always come exactly as you expect, when you expect. Ive been using them for 2 years now, and have never had a problem.
Anyway, the most important peice of hardware for any gamer, is the GPU. You do not need some super powered quad core processor, as none of the games use it, and when they do there is no noticable performance increace. And you dont need more than 2GB of ram (get ddr2 800mhz tho). What you do need tho, is a decient Dx10 graphics card. Take my system for example, It has a dusty old AMD X2 6000+, and 2GB 800mhz ram, and those two components are not the "recommended" for crysis and the likes. But because i have an overclocked 8800GTS to 600mhz core, i can run every single game on full. Imageine if i had an intergrated solution as Uncle sam oddly sugested, i would have a hard time running games whatsoever, let alone on full.
Lol on a side note, funny how windows is the most expensive thing on that list

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Edit: Sam, you can shave over $100 on your system, if you get it in peices from a site like scan.co.uk, or ebuyer.com. Dell is a ripoff. Im just giving you advice, i dont mean to rant.
CPU: AMD X2 6000+ 3.0ghz GFX: NVIDIA BFG Geforce 8800GTS 640MB OC-550mhz core RAM: 2048mb