Ok, not sure where to put this so I thought I would start here.  does anyone have or know of reference material that will let me know what can be upgraded (mem, vid card) without any major comp changes and what parts require a reload of the OS etc?  And any tips/techniques that can make the process less painful in the case of a full OS install.
 
Here is a basic example of what I am looking for:
 
1) GPU -  if compatible, unplag old one, plug in new, load drivers, done!
 
2) CPU - no idea. I know it can be done but what are the things to keep in mind.  ie: reload OS, make sure you have a new cpu fan because the lockdowns on the old one are screwed(?) etc.
 
3) Mem - open case, remove old mem, install new mem, restart comp and get BSOD because your OC and new mem are not playing well together
 
4) Motherboard - ummmm.......where to start with questions?
 
5) HD - ok, I know if I remove my old HD and put in a new one I have to reinstall everything.  What do I do if A) I just want to add a new one for additioanl storage B) want to change my main HD to a newer/faster one, in other words, do I have to reload all software or is there a way to make everything from the old go to the new? C) I add a 2nd, 3rd, or 4th HD and I want to set up a raid array.  Do I need to set them up then reinstall OS and all software etc?
 
I know these are realtively basic questions (which I do not know the answers to) but after having built my 1st system I am wondering how far I can go with it before it would make more sense to just start from scratch.  I did not include my comp specs because this is meant to be an "in general" type of question.  Basic assumptions would be P35/680/X38/780 age board, DDR2, mid range GPU and some type of Core 2 Duo.  
 
If this has been asked and answered in the past I apologize for wasting your time but I did not see anything in the forums.  Thanks in advance for the help.  
 
p.s. while you are reading and answering the questions above I will be out buying a new lawnmower.  So if you get irritated keep that idea in mind; answer noob questions or mow lawn, hmmm?

Depends on the OS. XP can sometimes survive a motherboard swap, most times it cannot.
 
Vista does much better here.
 
Motherboard is about the only thing that you could change which could require an OS reload. Everything else is just drivers.