Done previously when you buy a PC:

1.       Solicete performance of all PC Parts\hardware
2.       If there is no any consistence with one party to another party, these will be occurred a lot of problems.
3.       Accordance with your demand and budget you choose which PC you buy and also which parts you need.
4.       To make or buy a computer you need a Processor ,Main-board,RAM,HDD,Optical drive(CD or DVD ROM ),casing, monitor,Speaker,Mouse and Key-board.
5.       If you have a graphics editor ,games you need high level processor, Main-board,RAM and also need good PCI express .See the other post and get full explained about these parts.