ebuyer is a UK site that would work. I'm not sure about their prices though.
Motherboard: Gigabyte GA-990FXA-UD5 AMD
- This is an AMD motherboard taking both AM3 and AM3+ CPU's. AM3+ is the upcoming Bulldozer "FX" processors, the AM3 is the current generation. This motherboard will allow for both SLI and Crossfire. £156.51
CPU: Here you have quite a bit of choice. The current top-of-the-line AMD CPU is the AMD Phenom II X6 1100T Black Edition 3.3 GHz
(£153.84). That is a six-core CPU, but, unless you do media encoding, six cores may not be much of a performance increase over a quad since few games are optimized to take advantage of it. The AMD Phenom II X4 955 Black Edition Socket AM3 3.2 GHz
(£103.96) is also a fine choice. The new Bulldozer CPU's are drop-in upgrades, meaning you don't have to do anything aside from pulling out the old and putting in the new, that's it. If all you want is a CPU so you can power the system until the FX CPU's are out, you can get the AMD Athlon II X4 640 Socket AM3 3GHz
Case: Here you have two options based on how you use the PC. If you are more of a mid-level gamer who doesn't want the huge graphics cards or multi-GPU setups, a mid-ATX tower will work. The Antec 300 Three Hundred Case
(£49.88) is one such case. If you do want the large graphics cards, the NZXT Phantom White Full Tower Case
(£105.47) is good.
RAM: Corsair 4GB DDR3 1333MHz Memory Module
- I suggest getting two of these for a grand total of 8GB. Over the next few years, you may need the extra RAM, but if you are keeping costs down, you can make do with just a single stick. £32.34
Power supply: If you only want a single graphics card, the Corsair 600W GS Series PSU
(£67.84) is a nice choice. If you want SLI or Crossfire, the Corsair 700W GS Series PSU
(£76.45) or the Corsair 800W GS Series PSU
(£80.72) are suggested.
Hard Drive: Western Digital 750GB 3.5"
- This should be plenty of space. £40.89
ODD: Sony Optiarc AD-5260S
- This is a standard drive. £14.91
Graphics Card: Here you've got choices. The XFX HD 6850 1GB
(£117.99) is a solid mid-level graphics card. The HIS HD 6950 IceQ X Turbo 2GB
(£209.99) would be a high-end graphics card and the next step up. The Sapphire HD 6970 2GB
(£256.00) is the fastest single GPU card AMD has, but is only slightly more powerful than a 6950.
OS: Windows 7 Home Premium 64-bit OEM
- You need this. £67.07