Like most 49er fans, I'm loathe to quote Grant Cohn, but he did actually provide meaningful insight recently: http://49ers.pressdemocrat.com/2013/03/inside-the-49ers/the-49ers-defensive-scheme/
On base downs, the 49ers use a nose tackle, but that does not mean they use a 3-4. In a 3-4, the nose tackle lines up directly over the center in a zero-technique position, and the ends line up on the outside shoulder of the offensive tackles in a five-technique position – creating three "bubbles" at the line of scrimmage. The 49ers don't do this. The 49ers base defense is a 4-3 Under, a variation of a four-man line. The defensive ends are not five-techniques. The ends are reduced, or "eagled down," over the outside eye of the guards to three-techniques, and the outside linebacker – Aldon Smith – essentially is a stand-up defensive end.
The nose tackle is a one gap player. He lines up between the center and guard in one-technique position. The strong safety usually plays the eighth-man-in-the-box concept, even from depth, just so nobody along the front has to be a two-gap defender.
Glenn Dorsey, the 49ers' newest defensive tackle, played five-technique for the Chiefs and was not successful, but the 49ers will not ask him to play five-technique. They're going to ask him to play three-technique or one-technique, classic 4-3 positions, the positions he played in college.
In base downs, Dorsey will play the one-technique. He's the unquestioned started, IMO. The camp battle will be between Ian Williams and Quinton Dial for the back-up position. Williams likely has an edge as a pure, 0-technique NT, but that's not really a fit for our defense. With our emphasis on versatility, our back-up NT also needs to be able to play as an impact DT as an interior pass-rusher in our 4-2-5 scheme. I think Dial has the edge at back-up there.
The biggest change is that Dorsey can play all three downs, if we need him.
So our base 3-4 line is Smith, Dorsey, and McDonald. Our back-ups at DE will likely be Carradine, Dial, and Tukuafu. Our back-ups at NT will be Dial and Williams.
In our nickel 4-2, our line is Smith, Smith, McDonald, and Brooks. Dorsey and Carradine can rotate in (and Carradine can play inside or outside). Lemonier and Haralson and Johnson are depth for the outside. Dial may be part of this rotation as well, though he and Williams are less likely due to the need for a pass-rush in this set.