My PC cost £400 when I first bought it......although its worth more as I upgraded.
Get a decent motherboard, going to cost you around £100
CPU....I got a AMD 3800+ recently for £89
HD: depends, 80GB is good enough for me and was quite cheap. Their around £30
For ram, you can pick up 512mb sticks for £15, they're just crap for overclocking, but work fine otherwise
GFX card: I've got a 6600 GT, and that plays most gaes on reasonably medium-high settings. But try and get the best gfx card you can, as they'll soon go out of date (ie geforece 7)
For an optical drive, really matter of choice. More features = more money, but I'm sure a basic DVD drice would be about £20.
You can probably pick up a cheap case for £20-30 with PSU included.
I'll scout around for some links in a minute