Overclock.co.uk and ebuyer.com are two websites that sell PC components to the UK.
£400 is a bit low, you may want to increase it by another £100. I priced out a build over at Overclock, a mid-range AMD build.
Motherboard: Gigabyte GA-970A-D3 AMD 970 AM3+
- With an AM3+ FX CPU, you should get a 900 series based motherboard chipset. The 970 is the basic, cost effective variant. £62.99
RAM: Corsair 2GB Value Select 1333MHz DDR3
- I would buy two of these. £8.51 per stick.
CPU: AMD FX-6100 Black Edition 6 Core 3.3GHz
- This is not the fastest, nor slowest, FX CPU. £82.98
HDD: Seagate ST1000DM003 Barracuda 1TB SATA3
- This should be enough for most uses. £53.99
Case: Gigabyte GZ-X2 Black ATX
- A good mid-range case, don't expect to cram huge top-tier graphics cards into it, however. £29.99
Power Supply: PowerCool 650W PC-650AUBA
- I'm not familiar with this brand. This is enough to power most graphics cards. £38.99
ODD: Samsung SH-S222BB 22x SATA Dual Layer DVD-Rewritable Black
- This is a standard DVD drive. £11.58
GPU: Asus 1024MB GDDR5 AMD Radeon HD 7750
- This is a mainstream graphics card. It can handle gaming well, but lacks the performance of the 7800 and 7900 series. £76.50
OS: Microsoft Windows 7 Home Premium 64Bit
- You need this. Windows 8 will be out in about three weeks, you may want to wait until then. £67.50
Grand Total: £458.93, all of these prices include VAT.
If you want to save a bit of money and do not game, you can remove the GPU and swap out the motherboard and CPU for the components below.
Motherboard: Asus F1A75-V PRO AMD A75 Socket FM1
- AMD's new FM2 CPU's are out, but there aren't many in stock. £78.90
CPU: AMD Llano A8 X4 3850 Quad Core 2.9GHz Socket FM1
- This is AMD's APU design that has a graphics card on the CPU itself. The GPU isn't fantastic, but can handle all standard PC activities well and light gaming. Price £72.95