Its a good deal for them. They are basically giving up a 2nd round pick for a 3rd and moving up in the 1st.
Instead of having 2 first rounders, a 2nd, and a 3rd, they get two HIGH 1st rounders, and two 3rds.
Having the two high firsts could give them everything they want. At pick 20, there is pretty much no chance that either Sanchez or Stafford are there, and there is also a slim chance that one of the top few OT's will be there. But, at pick 11, its very very possible a QB is there, and if not, one of the top lineman.
Moving up from 20 to 11 gives them much more flexibility with the number 1 overall. They can have their LB they want in Curry, and still grab one of the top tackles they couldn't get at 20. And... they get another 3rd rounder.