Very well then, for a decent sstem that'll last 3 years and upwards...assuming you download things etc and very much into gaming and want the best quality for reasonable money.
24" monitor, because in the long run its worth it (i have one it makes crysis look sssoooooo good

) http://www.ebuyer.com/product/159782 £152.58 and thats CHEAP. mine was £312.
Antec 1200 gaming case for awesome cooling. http://www.ebuyer.com/product/143852
£136.99 there but you can get the for £100. You want this not only for the size, the fans and the psu, but also for the case design and removable washable filters.
(so £252.58 so far...)
2x dvd RAM drives because we all know 2 drives are vital. http://www.ebuyer.com/product/101321
£15 each so £30
(£282.58)
processor fan to keep things frosty. http://www.ebuyer.com/product/105994
Artic coolers are immesely good for a fan based cooling system. I have this in mine and it works briliantly. £14.86
(£297.44)
Decent graphics card. http://www.ebuyer.com/product/139641
I've got an 8800GTS aswell, works very nicely and plays crysis well enough. £195.73
(£493.17)
main program drive. http://www.ebuyer.com/product/164274 500 gigs works nicely for all those fat games. £40
(£533.17)
Backupdrive for storage incase it goes tits up and you lose all your downloads etc. http://www.ebuyer.com/product/164282 £60
(£593.17)
processor: quad core http://www.ebuyer.com/product/164969 Decent enough.
motherboard: http://www.ebuyer.com/product/145747 bundle deal with processor £224.12 for both together.
(£817.29 oh look we've shot right over £600)
RAM: http://www.ebuyer.com/product/142387 I *think* that one works with it, 2am so forgive me if its the wrong type. We'll need 4 gigs because prototype by itself needs 2 gigs to run (according to the box and we wanna be able to point at the box and laugh in a superior manner) £35.64
(£852.93)
power supply: http://www.ebuyer.com/product/151287 we want one with a bit of grunt for further expansion in the future, so we'll go with a 750 watt. £31.05
(£883.98)
soundcard: (optional since oboard sound is pretty decent these days) http://www.ebuyer.com/product/121761 Soundblaster didnt let me down before so we'll go with it this time, £37
(£920.98)
mouse: standard MS optical (because you honestly dont need anything more complex unless you're a gaming addict with OCD) http://www.ebuyer.com/product/106194 £14.82
(£935.80)
keyboard: same reasoning as the mouse. http://www.ebuyer.com/product/160332 £9.78
(£945.58)
Headphones/earphones (because you dont need speakers really) I'd give you a link to mine since they are epic quality..but they are discontinued -.- fuckers. But in reality it's your hearing not mine so its personal choice/trial and error.
So lets say about £15 for a decent set.
(£960.58)
Mic: Since you'll probably be using TS, vent or some other communication program you'll need a decent mic. a USB mic works wonders and has a huge range. http://www.ebuyer.com/product/54130 £16.42
(£977)
network card: http://www.ebuyer.com/product/83820 since you need it for interwebs...£10.84
(£987.84)
OS: since we've got 4 gigs of ram we're gonna need 64 bit windows. http://www.ebuyer.com/product/123067 £82.31
(£1070.15)
USB Hub: Becase we all know how annoying it can be to wrestle with cable at the back of our rig especially when we have lots of usb devices. http://www.ebuyer.com/product/130521 £5.85
(£1076)
Thats all i can think of just now so i've probably missed something. But add on the prce of cables, shipping and it'll take you over the £1100 mark.
As my brother once said, if you are going to build a system, build one that'll LAST. I'd say 3-5 years before you cant play the newest games is a pretty good deal right?