Before you go flashing roms, ask yourself why you are doing it. For most people, stock is the best option as it's pretty worry free.
You can change the look off your phone without risking bricking it.
why do you want to overclock? If it's just so you can say your phone is fast then that it's plain stupid.
Most people who change roms and kernels do so because they know what effect they are after and know what they are doing. The questions you have asked tell me that you don't. Not trying to insult anyone, just want to make site you are doing it for the right reasons. If you truly do want to do it and learn more, read the hacking forums THOROUGHLY, them also go over to the xda developers forums and read those.
Sent from 1976 utilizing a 1.21 gigawatt connection. Yes, bionics were around then, they were still just working things out.