Galaxy S Advance Android update
Miui en Cyagenmod are so called ROM's. ROM's are like modified versions of the original Android OS. They are built for various reasons, one of them being to add more features to your phone that aren't available in the stock OS (you could see those as your updates). Just Google about it because there is tons of information available about the particular subject.

The other questions are already answered but about the warranty. Most ROM's require your device to be rooted. I guess that rooting will void your warranty but then again, there's always something as UN-rooting.