In terms of
CPU performance, It seems that the i7 860 is slightly faster than the i7 920.
IMO, the big difference comes with having or not the integrated PCI Express controller:
- The i7 860 will provide "only" 16 lanes, so you are "limited" when it comes to multi-GPU setups
- The i7 920 does not have an integrated PCIe, so you can make full use of the 36 lanes in the X58 chipset
Another difference is the triple channel memory controller available in the i7 920, compared to the dual channel memory controller in the i7 860.
I would personnally go for the i7 860, since it is more cost effective and I don't use multi-gpu's.