RE: is i7 870 going to work on dell inspiron 580?
The Inspiron 580 has been tested with the below processors.
Intel® Pentium™ G6950
Intel® Core™ i3 - 530, 540
Intel® Core™ i5 - 750, 650
Your findings in relation to the processor being compatible is correct. However, since the processor has not been tested on this platform, I am unable to assure you 100% flawless functionality.