£450 should buy you a decent base unit for game purpose. Tiny.com click here is also a possibility.
The most important part for game is graphics card, so get ATI 9600 or better, or Gford 5600 or better. A ATI 9600 will cost less than £70
The processor speed is not very important, but get as fast as you can, AMD processor is favoured by gamers due to its performance/price ratio.
HDD: 120GB or more
Get a DVD burner if the money is enough.