How will the motherboards cost less? This is a hackintosh, he wants to run OS X- he's going to be limited by what he can actually use, much more than by cost.

As for your assumption that the 860 is faster out of the box, it depends on what he intends to do with the machine (yeah, same old argument, but as you'll see at the linked test at the bottom, with a few caveats). There are many cases for which the 920 would be preferable, and it is an all-around superior platform to the 860, hands-down. As the prices are so close, even if he's only going to be doing everyday consumer-level stuff, it's still a better choice, because for the everyday stuff (word processing, iTunes stuff, etc, etc) there'd likely be no noticeable difference in performance and the prices are so close (so go for the better offering). If he wants to do things such as video encoding or heavy-lifting with Photoshop, the 920 will likely best the 860 by a decent margin.

Intel's Spec Stuff

Here is a good comparison, not just between these two processors, but alongside the i5, Core2 Quad, and AMD's offering (AMD... HAHAHA, whatever :oP ). Their tests would appear to reveal that the 920 is, per cycle, faster than the 860. However, they seem to prefer the i5 for overall value (price-to-performance ratio), and that may be something to consider.
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