I guess that you are installing via MS update which is a smaller download but will still take time. The other way is to download the full SP3 file. click here
You are not required to backup but you should consider it as a priority in case anything goes wrong in the future.
Should be OK to install - SP3 sets a restore point first and keep your fingers crossed that all goes well. Norton IS 2008 works fine most recent versions should be good too.