Unless you are rooted (now someone correct me if I am wrong, because this is my first Samsung android phone (always had HTC's)), you really can't clear the cache manually. You can do a factory reset before you flash, which is about as close as you can get. Once done, then you can flash this ROM using the guide above.
Now, Titanium Backup. For the type of backup/restore you are asking about, you need to be rooted. I normally only backup Applications and App Data, NEVER system.
Now, when going from GB to ICS, I would refrain from backing anything up with the intention of restoring it. Completely different file system structure and OS Frameworks. If you can backup the data natively in the app, do it that way. Doing backups and restores can add a layer of instability that may have come over with whatever is in the app/data. If the app itself does not have a backup/restore feature, I tend to not back it up.
BE WARNED: EVERYTHING you backup to "SD Card" in the apps will be backed up to the INTERNAL SD Card. THIS WILL BE WIPED WHEN UPGRADING!!! Back it up to your external card or PC BEFORE you even start!!
Hope this helps.
Thanks!