Blythe, our current center is a savvy vet, smart at making the line calls, and is a rubbish blocker. The draft has a few centers that have been mentioned in the early rounds (2-4), or we could again try for a free agent vet. Similarly, we could use an upgrade from Jackson at right guard. The guards this year are underwhelming, especially if you watch some of the 1x1 clips from the Senior Bowl practices. The best of them are being regularly driven right back into the pocket (and the comment is "great block").
My leaning is to take a drafted center and sign a free agent guard. Two rookies worked last year, but let's not push our luck. It is important to have a center who is not a converted guard or tackle. He needs to have experience making the calls and recognizing defenses.
The best of the centers, Van Pran, who cBob mentioned, went back to school, but the class is still strong. A third round pick would give us a guy who could stick.