Will a Intel Core i5 3570K Unlocked Quad Core Processor LGA1155 3.4GHZ Ivy Bridge 6MB Retail (http://www.directcanada.com/products/?sku=12200BD0768) be compatible with a Dell Inspiron 580's motherboard? I'm looking to replace my current i5, 3.2ghz dual core with a quad core 3.4GHz.
Nope, it won't work, since you're right about the mobo being LGA 1156 (needs to be LGA 1155 for the i5 3570K, obviously). There's only one "generation" of the Inspiron 580 (that post is talking about second gen Core (Sandy Bridge)).
Thanks DJDeCiBeL and malmental; I wasn't sure and now I am. The 2.8ghz quad core i5 1556 was what I was planning to get originally but hadn't known which sockets I have for my Inspiron 580's CPU. All's fine now.