Try click here they have the 939 version of the AMD64 3000 (inc HS & fan) for £124 inc VAT on pre-order.
As for mobo, try click here thay usually have a good few reviews on 939 boards. The new Nforce4 chipset is due out soon, looks very good but also could be handy for driving down price of current boards.
A couple of months ago I built a base unit with a 754 pin based AMD64 2800, 512 MB memory, 128MB Nvidia 5700 card (DX9), 80GB sata HDD, XP pro & CD/DVD-RW for £400. No monitor was needed but they are now even cheaper since then, so you should be able to build a good gaming rig for £900.