Rooting will not change or delete any apps or data. Rooting simply gives you access to the root of your phone, allowing more control over your phone. personally, I recommend framaroot
, a one-click app to root the i777 (and many other phones).
Once you're rooted, I would recommend getting titanium backup, which would allow you to:
backup all of your apps and their data.
backup sms & mms & wifi data & ...
get rid of system apps that you don't want or need.
...and a host of other awesome items.
I don't know whether how updating to JB through kies will affect your app or system data, but I'm fairly certain that it won't keep much of it.