Please explain the difference between these CPUs...

This X5680 is 920 bucks...

http://www.allhdd.com/index.php?target=products&mode=search&subcats=Y&type=extended&avail=Y&pshort=Y&pfull=Y&pname=Y&pkeyw ords=Y&cid=0&q=592168-B21&src=ggl&gclid=CO_Y6p7w9rYCFQE6Qgod1hYAiQ

And this x5680 is almost 1700...

http://m.tigerdirect.com/applications/searchtools/item-details.asp?EdpNo=6047852&SRCCODE=WEBGOOPA&cm_mmc_o=mH4CjC7BBTkwCjCV1-CjCE&gclid=CNrXh9Tw9rYCFUzhQgodRnwAZQ

So, why the drastic price difference? I'd love to find a cheap 4,1 8-core MacPro and put these in there.