i have just built a marvellous PC for my brother for:

Athlon II x3 at 3.2GHz - £58
Asus 880G/USB3 m/b - £68
4GB DDR3 at 1333MHz - £48
Nvidia 460GTX 768MB - £114

grand total = ~£280

a fitting rig for Guild Wars 2 now that he is the big cheese at Zam

if you want something more bling, what I have:

Phenom X6 at 3.2GHz - £180
Gigabyte 890FX UD5 - £140
8GB DDR3 at 1600MHz - £105
ATI 5870HD 2GB - £300

after xmas however a number of cool things will happen:
Intel release sandy-bridge which means CPU prices for everyone goes down.
ATI release the successor to my card; the 6970

up to you