I have done this myself numerous times in the past, build a PC from scratch or upgraded a PC with new hardware, however for some a PC is still a mystery box that just works.
Anyhow, I stumbled across this neat video that take you through the process of building a PC – so if you ever had the urge to get on-board with a project like this you really should watch this video, it gives you some great pointers and also give you some idea on whats involved. A word of caution though, this guy know what he is doing and is 100% sure all the parts will fit, that in my recollection is sometime where things gets tricky (when things wont fit or work together).
Enjoy..