Core i7 920 vs Core i7 860: The Platform
While the Core i7 920 and Core i7 860 use the same basic architecture, they use different sockets. The Core i7 920 is an LGA1366 processor for X58 motherboards. The Core i7 860 is a LGA1156 processor for P55 and H series motherboards.
There are two main differences which result from this. One is that the Core i7 920 will be used with tri-channel memory, while Core i7 860s will be used with dual-channel memory. Tri-channel needs to be paired in groups of three for optimum performance, rather than the traditional two. Tri-channel memory offers higher theoretical memory bandwidth, but I'm not aware of any commonly used program which can take advantage of this.
The other difference is that X58 motherboards have enough PCI bandwidth to handle two video cards running at full 16x PCI-Express speed, while P55 motherboards do not. Most P55 motherboards will instead split the PCI-Express lanes when two video cards are mounted into a 8x/8x configuration. This reduction is not as dramatic as it sounds and does not mean the video cards will be running at half speed. There is, however, a measurable difference. Two video cards running on an X58 motherboard will likely have a roughly 10% performance advantage over two video cards on a P55 motherboard.
These benefits both favor the Core i7 920, but there is a price. The X58 motherboards are on average more expensive than P55 motherboards. While a quality P55 motherboard can be found for around $100, most decent X58 motherboards are closer to $150. Also, the need to use tri-channel memory makes buying memory more expensive overall since you'll be buying in sets of three instead of sets of two.