As far as signing a FA I'd mostly look at S since drafting one seems like a major crap shoot. Byrd is one of the best coverage safeties in the league so he'd obviously be a great pick up though probably pretty expensive. Some other guys out there as far as I know are Whiter, Pollard, Ihedigbo, Mikell, Woodson (lol), and Ward. I would look into TJ Ward from the Browns, he's not really a big name guy but he's a good all-around type of player that would be a big improvement.
ILB FA looks like crap again this year so I wouldn't count on that too much.
OLB outside of Orakpo and Worilds is pretty thin. DE as far as edge rushers type is much more plentiful in case we happen to move on from Dom and change schemes.
At CB we really aren't hurting too much that we'd need to reach into FA. I think everyone would like to see Shields re-signed though.