Just a quick head's up to folks using this mobo:
The latest Mobo firmware (maybe earlier ones too) support Xeon 5600-series hexa-core chips out of the box. So does Mavericks. You'll want to reset the BIOS to optimized, because voltages and a few other things will change, and then go in and make sure you have the right things set for OS X/Hackintosh. But not need to reinstall anything else; it'll boot right back up into your current install.
For reference, I'm running a low-power L5639 hexacore overclocked to 3.8 Ghz on all 6 cores, and it scores 17,100 geekbench and tears through video compression and other heavily multithreaded tasks.
I just picked up an X5650, which should let me get a safe 4.2 Ghz OC (because it has a 20X+2 multi, vs. the 16x+2 Multi on my current chip) as long as it's a solid performer. These chips OC better/further, at lower voltages/heat than my previous i7-950.
Best part? They're ~$75 on eBay, as tested pulls from working server farms. That means they've seen plenty of low-power use, no OC'ing, and are likely very, very healthy.