Well, I think this thread is a good place to start for ideas.
http://www.themanadrain.com/index.php?topic=37992.0Beyond that, 4 Zorbs are superfluous; 2 are all you will ever want. Also, Mox Diamond is some good in a deck like this.
Tariff is cool but I would sideboard it rather than maindeck it, based on your description of its use.
Planar Birth is not a card I am thrilled to draw multiples of despite its many tricks, so I would cut at least 1 copy. I've never really played with Cataclysm, so I can't really comment on it other than to say it looks interesting.
There's really no reason for your deck to fold to counter spells; at least, not in a casual environment. Argivian Find, Abeyance and Replenish are the way to go.
Here is a decent link for general ideas. I don't know if you have read this primer before or what, but it might be a good refresher.
http://www.casualplayers.org/article/get.php?action=getarticle&articleid=344