A computer is made with different parts; therefore you have to know which parts do which work to have a usable computer. Remember that the components of a computer depend on which computer they will be used for. Similarly the purpose of the computer will also dictate which components you have to use.
The CPU dictates the motherboard you have to use; therefore you have to choose the CPU before choosing any other component. The two CPUs from Intel and AMD work similar, but AMD is ideal for bulky work. Your budget also dictates the kind of the computer you will buy. The computer of midrange is the cheapest, you can get it on an average for $700, the processor with more capacity can be gotten for above $1000.
Before choosing the motherboard, make sure that it is compatible with the CPU you have already bought. From there you will check the other features you would like. If you do not want to buy an extra sound card or video cards, make sure that the motherboard has them in-built. You should also consider the interface, the memory sizes, the expansion and its speed. And make sure that they have the best chipset. The RAM of your computer should be 256 up to 512MB if you want to use window XP but for an average work it is better to use 1GB for the speed. Make sure that your computer has a compatible DIMMS with a good CAS latency.
The hard drive should be considered based on the size, 120GB is the average used for all people. But if there is a need to store more video or pictures, make sure that you have a hard disk of more capacity. The hard disk interface must fit the motherboard and the one with the serial ATA (SATA) is better than the one with IDE. Now you can get the SATAII whose speed is the double of the SATAI.
If you like gaming and you intend to use your computer for gaming; you should buy a video card. The most important feature of video card is its interface, the average one is 6600 but if you have the money, you can go for the expensive 7800.
You also have to think about the CD drive with a DVD burner if you think you will need it. For the power supplier, the minimum should be one with 300W but the high end computer should be with 450W PSU. The most important factor is that all the components of your computer are compatible, you may consider buying the same model for all the components.