Yeah I wouldn't be too worried about CABNE/CABYE/CABGE. I've seen Cabbages that could do just as well as the other two, and CABNEs that could only OC as well as the supposed cabbage-level overclock.
Now I suppose if you want to play it safe, you could buy the Opteron from somewhere you don't know the stepping is (Pretty much any retailer), and you most likely won't get Cabbage.
Or you could order three Opterons total from three different retailers, test them all, sell off other two.