The suggested +spell thingy used to be around 950 ish, some more could do good too, but wasn't a big deal. Now, with all the +base damage thrown around like candy, you might want to rethink that. But I don't feel qualified to give a precise number, since it all depends on your gear anyways.
From the top of my head, I've got something like +25% base spell damage, and run at 800 +spell damage (admittedly low). I'm aiming for 1,2k +spell damage to run some test parses myself at the moment, but I don't have the easy access to uber gear that some of our fellow posters here have.
A good robe, easy to come by, is the T2 one, and work from there. And it's the "robe of the undying" fyi
