Presuming this is a hardware-only cost, you can spec a pretty nice (Athlon quadcore, 4G RAM, Iiyama 22" TFT) machine for less than £500 at http://pcspecialist.co.uk - I've used them in the past and their builds are very decent. If you need Windows, unfortunately that blows about 20% of your budget. This is one of the many reasons why Ubuntu is such a good and shiny thing.
Presuming this is a hardware-only cost, you can spec a pretty nice (Athlon quadcore, 4G RAM, Iiyama 22" TFT) machine for less than £500 at http://pcspecialist.co.uk - I've used them in the past and their builds are very decent. If you need Windows, unfortunately that blows about 20% of your budget. This is one of the many reasons why Ubuntu is such a good and shiny thing.