would be my first advice e.g: PC ADVISOR!.
Especially looking at some of the back-issues for "Build your Own PC"!
You match the CPU (Processor) of your choice and budget to the correct motherbord. But you have to know which processor is correct to fit in the motherboard. So we have AMD & Intel, AMD Processors being the cheaper on the pocket. But quality is sacrificed for this saving.
Current AMD CPU's are for Socket A motherboards.
I could carry on for hours on the subject but due to space restrictions here would leave you short of a lot of considerations and infomation etc.
Please feel free to mail me direct by clicking on the yellow colour folder thingy For Tunes. If you wish more guidance.
Read more magazines, hardware reviews, websites on the subject for build your own pc. Many for & against articles around. But the satisfaction and the end of the day you will be very impressed with your own work if you proceed.