cassettes/chains

1)  in most cases, no, but you might want to get a new chain with a new cassette to match the wear

2)  you should be able to check the specs of the particular RD - it does matter (e.g. short cage vs. long cage - I had to switch mine to go to 11-28 at the time)

3)  if you just replace it with the same groupset, you should be fine

4)  there are a TON of books and youtube videos on bicycle maintenance, etc.

As for the cassette, ya tighten that sucker hard.

Just went through all of this on my bike. I upgraded from a 6sp to 8sp cassette and derailer.

I found a lot of information on this website. Some of it antiquated, some fresh. The guy was huge in the bike community and passed away, the site still seems to be kept up and current by friends and family.

http://www.sheldonbrown.com/

Now before we get into all the questions lets look at what you are trying to do. Typically there are 2 types of cassettes folks are looking for, Racing or Climbing. Typically racing gears are going to be close ratio and higher geared (small numbers/diameters) for speed. Climbing gears for tackling tough climbs are going to be a little more spread out and lower geared (big numbers/diameter). This will also coincide with a matching derailer, or you will end up missing some gears which maybe a acceptable sacrifice, sometimes its physically not possible since the derailer will actually crash into the larger gear diameters.

For instance my lowest gear is a 25 and my chain is already set tight, my derailer will handle a max gear of 28 and I would have to add a couple links to my chain to pull that off. If I wanted to go to say something like a 32 I would need a matching derailer and probably want a matching length chain to go with it. So its totally possible to have 2 complete setups, but just be aware you will want to change out all 3 components at once to make a smooth shifting setup. This all assumes the same speed.

1) chains will typically stay the same width within a range of speeds, although length will typically change depending on gear ratios(size) and derailer.

2) derailer will have to be the same speeds as the new cassette, also lower gears (bigger number, bigger in diameter) will require a longer derailer cage.

3) Not sure what you mean on this question other then its best to keep everything matched together.

4) Check the link I posted above and google search.

Good luck with the research, It took me some time but I was able to get my setup working flawlessly on my own without the bike shops help. I had them double check the work and change out my cassette but that was it. My upgrade was not much of a upgrade but it was what I had to work with and it did make a difference. I am able to tackle a category 1 climb on a 12-25/38-53 gear set and yet I was able to maintain 25mph over several miles with a average speed of 19.8mph over 16 miles. This is my sacrifice on a 27yr old road bike. Now I need to work on better brakes, lol!

Also this information is as I understand it and what has worked for me, so if I am not correct on something feel free to correct me.