Re: Help on looking for parts?
Definitely a good idea. I'm in the US, do you have a local chain or online site that sells PC parts that you've heard about or want to try? Some other UK folks may chime in as well, but I always like to customize a PC when the price cap is about as much as you suggested.
AMD based - Socket AM3 board, MicroATX should be fine, Full ATX if you can find one cheaply, and be sure it has onboard Radeon HD video - you can save some $$ and still play games.
AMD Athlon II based processor, the Phenom II's aren't worth it at this price point for the relatively little performance increase (L3 cache performance on AMD's Athlon II / Phenom II architecture is nil) but I'd get at least a Quad Core if you can swing it. Otherwise a Triple Core, or a dual core if nothing else.
At least 4GB of RAM, DDR3 brands I trust are G.Skill, Patriot and Corsair (in that order)
Hard drive - you can get 500GB for as little as $30 here in the states
DVDROM - Most sell online for less than 20 bucks (again, US$)
Case - pick one, doesn't have to be snazzy as long as it's got the right slots for everything
Power Supply - Antec's got some great higher power units for an inexpensive price - 450-500 watts should be fine for an Athlon II build with integrated Radeon HD graphics
OS will be the harder part - if you've got a deal at school, use it, otherwise OEM Microsoft Windows 7 Home Premium x64 is about $89-99 here in the US (online)
You can also check to see if there are any "parts PCs" vendors on EBay - you can piecemeal a system from one seller, I've seen it, but it's rare. (i.e. that is, get all the parts from a seller who has assembled them together before, but ships the parts to you in packaging instead of inside a PC)