As Belichick says, it isn't about just one move. You can't build a team with one player or one good draft pick; it's a lot of things and you have to make consistently good evaluations across the board. When I look at the secondary, I think the Pats are much more than one upgrade away from being good, or even average.
Since the linebackers are so run-focused, you'd think the Pats would focus on strong cover safeties. When you have Mayo, Spikes, and Hightower, do you really need run stopping specialists like Chung? I don't think it's a coaching issue, but rather it's a personnel issue. Horrendous draft picks (Dowling, Wheatley, Wilhite, etc, etc.), poor free agent signings (Gregory, a bunch of DBs that just get cut,) and a lack of premium on the position. When is the last time the Pats actually spent money at the position, including their own re-signings? Ever?
On the bright side, I've been a McCourty defender for awhile. Based on Belichick's comments about DMc, and the playing time, I'm willing to bet that Belichick considers him one of the three most important guys on defense, and I think that most of us don't understand his value to the secondary.
Anyway, I think Talib might make us slightly better, which doesn't solve the major, major issues in this poor secondary. These guys back there are clueless; they aren't even close to being a good secondary, let alone a championship secondary.