First off you need to honestly sit down and work out what the PC will be used for. The usage determines much about the characteristics of the components you buy for your PC. Alternatively you can buy top quality of everything and have a PC that will do anything but you may end up spendin a lot with power or features you never use.
There is usually a trade off between what you want and what you can afford so you have to understand how to compromise on what you want and what you buy. Buy what you NEED, not what you want.
A PC is 15 to 20 components, and if you decide to buy a better this, and a better that, than you actually need - you end up spending a lot more money than you expect.
Watch out for the reputation of companies and manufacturers with regard to post sales support, RMA etc.
The starting point is the processor - decide what you need.
Then work out which cooling solution, including TIM cleaner and thermal paste you need. Ensure any heat sink will actually fit in any case you decide to buy.
Processor choice points you towards a certain family of motherboards. Look at motherboard specs and again buy one sufficient for your needs.
Motherboard choice points you towards memory. Ensure the memory you choose is suitable for the motherboard and the cost is balanced with the cost of the motherboard and processor. No point putting top quality fast memory in a system with budget processor and motherboard, and vice versa. Good quality memory can be surprisingly cheap at the moment. Make sure you get enough. The total amount USEABLE is determined by choice of OS, no matter how much you actually physically install.
The purpose of a case is to physically support the components and aid cooling. There are secondary aesthetic issues. Spend as little or as much as you like but make sure it is fit for purpose and not just a pretty face. You may end up looking at it and using it for a decade so get one you like the look of!
Buy a good quality power supply from a reputatable manufacturer. Don't skimp on this - the power supply is one of the most critical components in the PC. Ensure it delivers sufficient power. As a rule - the heavier in weight a power supply the more likely it is to be a good one. Try to ensure it will fit in any case you choose without fiddling around removing power supply fan grilles etc.
Keyboard and mouse are a matter of personal choice and comfort. Easy to replace if you get it wrong.
Soundcard and headphones/speaker system is a matter of peronal taste - look at a hi-fi magazine to help you understand what to look for with respect to sound quality. More important for musicians and gamers.
Good quality hard drives are cheap as chips now - OC forums give good advice.
Optical drives are also very cheap unless you want Blu Ray. Look at OC forums for recommendations around £20.
Do you want floppy drive, card readers, or a combines unit for both? Nice and cheap in any case.
Screen and videocard should balance with one another. Decide on a screen and from the screen specs (native resolution) decide on a video card. The actual video card will be heavily dependent on usage - games or not, Directx level needed. office work, etc. Ensure the video card will fit in the case you choice - some long ones will not, or will block key areas of the motherboard, or will block access to drive bays etc. For photograpic work a little care might be necessary in screen choice.
OS - Windows 7, Vista or XP. Which version of DirectX do you want, do you have any applications that demand a specific OS? Don't buy OEM licence if you intend to use the OS in later years on a different PC - get a retail licence.
Hope this brain dump helps.
