It looks like you haven't built a computer in a while- for example, you DEFINITELY want DDR3 RAM and you almost certainly want a 1TB HDD if you're gaming (it's not much more expensive than 500GB and would be a small part of your budget). Frankly, you should consider a RAID array (simultaneously back up your data on two identical drives). For reference though, start here:
Logical IncrementsYou can probably build one of the computers straight off that example list, throw in a sound card and a WiFi card, and be very happy with it. In my opinion a thousand pounds is also plenty of money- past 800 or so you really start to see diminishing returns and unless you REALLY insist on having the latest stuff, it's not worth it to spend more compared to upgrading later.
Within your budget, you're probably already looking at a close to top-end Nvidia video card (Nvidia owns the top of the market) and a good i5 processor (you don't really need more), as well as 8 gigabytes of DDR3 RAM. It's important to think about what the slowest part of your setup will be, but assuming it's not processor-limited (shouldn't be with a recent i5) a 180 pound Nvidia GTX 760 graphics card will get you a consistent 60FPS at maximum settings and 1080p resolution in Tomb Raider or Skyrim, and 45 FPS in Far Cry 3 or Crysis 3.