Is there a way to `tar` extract without clobbering

I have difficulty understanding what you want but if you made a tar file of your whole system I believe you made a very large tar file that has not a real purpose unless you completely restore the tar file. If you wanted to save your own documents you should have made a backup of those documents and nothing else.

After saying that tar has several nifty options ...

--keep-old-files

This will not overwrite existing files.

--keep-newer-files 

This will keep all files that have a newer timestamp as is.

--no-overwrite-dir

This will leave the metadata of an existing directory as is.

A manual for tar can be found here.

But I would advice extracting the tar file on another medium and manually restore those files. It is the safest method.

From tar(1):

 -k, --keep-old-files
       don't replace existing files when extracting

Seems clear to me. If files in the tarball are stored relatively to your old homedir:

tar xvkf backup.tar.gz -C ~