If the update works as it should (which is very likely), then you will still have all your apps and data. However, since some apps may have been upgraded in 4.0, their setting options may change. So after the upgrade, it's a good idea to check your settings.
A few apps(usually games which no longer get updates) are not compatible with android 4.0 so you might loose 1 or 2.
However, it's always safest to backup in case something goes wrong. Seeing as you will be using Kies for the upgrade anyway, you might as well use Kies to back up as much of your data as possible. You can backup contacts, sms, calendar, music, photos, settings and more.
I'm not sure if Kies backs up your Downloads folder. If it doesn't you will need to manually copy that folder to your desktop.
If the upgrade deletes your apps they will be reinstalled by Google Play as soon as you login. Your apps will be backed up on Google Play, but the settings and data will not (usually). So you would loose your game data, for example.