I would assemble the PC as you can target the budget better!
This is what I would get:
Phenom II X4 940 ~ £144
Asus 760G motherboard ~ £52
1TB Samsung Spinpoint F1 hard disk ~ £66
SATA DVD rewriter ~ £16
Asus HD4850 ~ £84
The total including postage is around £374. If you have 20+ posts on Hexus you can get free postage too!!
The other bits:
Antec Mini P180 ~ £55
OCZ 500w PSU ~ £45
OCZ 4gb RAM ~ £34
The total cost from Ebuyer is £134 with the free postage available. The total cost should come to around £508 and
IMHO is a solid build for the price.
I would use Windows 7 RC1 until March 2009 and buy a retail copy of Windows 7 then.
The money saved could go towards an OEM copy of XP if required but
TBH I would get Windows 7 as in a few years time Vista and Windows 7 will be better supported for games
IMHO.