Since this year's safety class is less than inspiring, I wouldn't expect the Niners to replace Goldson at this point. They are more likely to franchise him this year and replace him in 2013.
That leaves OL, WR, and CB as the team's biggest needs. IMHO, they will take the best player available at one of those 3 positions.
As much as I would like him on the team, there is no way the Saints let Nicks go... He is an absolute stud...and young too.
Also, I doubt the Niners would invest that much money in an OG to be frank, especially since they have a lot of good players to retain in the next couple of years. I doubt they sign any really high profile FAs actually, unless they think one player could put them over the top.