Building PCs - How do you Choose Your Components?

ATP asks: "It's been a few years since I built my current system by ordering parts from the cheapest vendors I could find. Everything went smoothly with very little research, and my system is still running great today. I'm now looking into building another desktop system, but I've become quickly overwhelmed by how complex components have become. In particular, motherboard/CPU compatibility are confusing (think: single/dual, socket 478/604/754/939/939pciExpress/940/LGA775, OS compatibility, memory types, etc). Is there a guide to the not-so-novice PC guru somewhere that would help me catch up on the developments in the last couple years?"