My desktop computer was state-of-the-art back in 2005, and it really meets all of my computer needs. I would like to keep it going for as long as I can by buying spare parts and fixing it as stuff goes bad.

I located nearly identical used CPU's as mine online, going for $10-$15. I have not located any used mother boards, however. I'm wondering how you even find an exact match for a motherboard, or do you just have to have one custom-built to match my old one, once it goes bad?

Also, how exact does a replacement CPU have to be, in order to use the same motherboard?