The 920 uses more expensive motherboards and triple channel memory.
It performs a little better (not much) with multiple graphics cards.
The 860 uses dual channel memory and is a cheaper build overall.
The i7 860 uses a P55 motherboard and the el cheapo ones can be had for under $100 although generally I dislike motherboards that are too cheap (less quality,features etc)
Also the 860 has a much lower TPD.
X58's boards will (It is claimed) support the über expensive future Gulftown CPU.
Not much difference but some.