If all you're looking to do is upgrade for the sake of saying you upgraded, save your money. You're not going to have a better gaming experience if you're already playing "at full settings without a hitch."
$120 might be a touch on the high side for the 192 core flavor. Checked ebay completed listings and looks like the 216 core varied from $110-$140. StarCraft II won't need that much power either. A much less expensive GPU will do the trick. So unless you want PhysX, I wouldn't worry about getting the 260. If you do, sell you're 4850 down and pay for half the 260, though that still doesn't seem like a good value transaction for you in the end.