At the moment its a toss up between two online stores.
Ebuyer.com and overclockers.co.uk
One has cheaper memory prices, some cheaper motherboard prices.
Depending on how much cash you will have and what you want.
You could easily build a AMD 6 core CPU, Pretty Decent Motherboard , 4GB DDR3 1333, and a Geforce 470 for about £600. Assuming you have everything else like Case, Hard drives and Optical Drives, PSU and of course a monitor.
If you dont you could easily add on another £250-300 again depending on what type of drives you go for. (And a cheap case of course).
OP should have been OS, as in operating system.
[Edited by DABhand, 5/9/2010 2:08:51 PM]
My cousin also suggested overclockers.co.uk so I spent a couple of hours checking out parts and comparing prices etc. My bare maximum budget is about £700 and that is absolute maximum at the moment.
If it does make a difference the OS im planning on running is Windows 7, but I thought there was more depending on what motherboard I decided to use rather than OS. Its possible however that I could sway towards to Vista purely because of how much it costs.