so thats roughly 800 usd? if so here is a build i'm about to make that should do fine.
this board and cpu combo has massive OC potential
Gigabyte EP45-UD3P
http://www.newegg.com/Product/Produc...82E16813128358
Intel E8400
http://www.newegg.com/Product/Produc...82E16819115037
Corsair 750TX
http://www.newegg.com/Product/Produc...82E16817139006
XFX HD 4870
http://www.newegg.com/Product/Produc...82E16814150394
Patriot Extreme ddr2 1066 (2x2gb)
http://www.newegg.com/Product/Produc...82E16820220397
and hdd and case are just your preference
Comes to $641.54 or 392.478 GBP So you have ~200usd for a case and hdd
you can't go wrong with
Antec 902
http://www.newegg.com/Product/Produc...82E16811129058
WD Caviar 320gb
http://www.newegg.com/Product/Produc...82E16822136098
and for an OS just get the Windows 7 RC and preorder the upgrade.