the differences between the 920 and the 860 are that the 920 has triple channel support but lower clock (2.66 vs 860's 2.8).
How do they compare?
Very similar at stock speeds for video encoding:
Core i5 750 - Core i7 860 and 870 processor review
AnandTech: The Lynnfield Preview: Rumblings of Revenge
bit-tech.net | Review - Intel Core i5 and Core i7 Lynnfield review
But if you overclock, the i920 is better:
bit-tech.net | Review - Intel Core i5 and Core i7 Lynnfield review