First thing is to make sure your i7-860 is supported on the QVL ( Qualified Vendors List ) of the Intel DP55WB Mother Board. How we do this is to look up the supported processors as show & YES the i7-860 is shown.
LINK:
http://processormatch.intel.com/CompDB/SearchResult.aspx?BoardName=DP55WB
Now that we are 100% certain the processor is compatable we then go on to research for the compatability of the 7870. We go to this link to see what the PCI Express supports as shown in the Features & Benefits list of the DP55WB Mother board
LINK:
http://www.intel.com/content/www/us/en/motherboards/desktop-motherboards/desktop-board-dp55wb.html
As Shown it has the following Expansion capabilities:
Expansion capabilities- One PCI Express* 2.0 x 16 bus add-in card connector
Two PCI Express 2.0 x 1 connectors
One PCI Conventional bus connector
In order to be certain one can also review the Expansion Options from the following link:
http://ark.intel.com/products/41162/Intel-Desktop-Board-DP55WB
Under the Expansion Options one will see this:
PCI Support 1
PCIe x1 Gen 2.x 0
PCIe x4 Gen 2.x 0
PCIe x8 Gen 2.x 0
PCIe x16 Gen 2.x 0
Now that we have researched this for compatability we see it has PCI Support 1 & the rest of the expansion ports are all gen 2. So will the 7870 with ◾PCI Express 3.0 x16 bus interface work in a ( One PCI Express* 2.0 x 16 bus add-in card connector )
ANSWER: YES, as it is BACKWARDS COMPATABLE!
The following link explains it somewhat:
http://www.tomshardware.com/forum/344877-33-will-express-card-work-slot
There you go, VARIFIED!
Good Luck