Theoretically, a combo of #13 + #17 could move the Niners up to #4, or even #3 with additional sweeteners. That would give them a shot at Eric Berry or Russell Okung. That would be ideal (theoretically). But neither the Redskins nor the Bucs appear eager to move down. And after the 4th pick, I don't see any players that the Niners would be crazy about moving up for in this year's draft.
If the Niners stay at 13 and 17, there should be at least two starting-quality players available.
In terms of immediate impact: ILB Rolando McClain and RB C.J. Spiller.
Historically, LBs and RBs adapt very quickly to the NFL because they rely more on pure athleticism/instincts rather than skill/technique/experience, compared to other positions. I like McClain's size (254) and instincts playing alongside Willis, while Spiller is a game-breaker with enough size to be a feature back.
In terms of long-term needs: OT Trent Williams and OG Mike Iupati.
The Niner o-line is in critical condition and needs a major upgrade. Free agency isn't going to help this year, so the draft is the only option. It usually takes at least a full season for rookie OLs to make a consistent impact, but they desperately need the talent.
If the Niners decide to trade down, they'll most likely end up with a bunch of players who will merely provide competition and additional depth to the existing players. What this team needs isn't competition; it needs a few uber-talented guys to come in, take control and elevate the existing guys to a playoff level.