You'd need a new daughterboard & both heatsinks, at least - the single & dual CPU boards are physically different & the singles have larger heatsinks which won't fit the duals.
picture from OWC
Changing the board also gives you 4 more RAM slots & a potential maximum of 128GB RAM, if you are running Mavericks 10.9 or later.
With earlier OSes, though it will take 128GB RAM [8x16GB in dual channel], OS X cannot use it all - it only supports a maximum of 96 GB [6x16 in triple channel].
Bootcamp into Windows & all 128 is a possibility.
The 5,1 should already have the firmware necessary to support the Westmere processors, the update is only necessary for the 2009 4,1.
BTW, if you are considering doing this yourself, be aware that standard off-the-shelf CPUs are expected to go in PCs not Macs & have lids. The ones for Mac are lidless.
You can make the lidded processors fit, but it's fiddly [& would need an entire answer in itself]
Refs: Everymac, MacSales, OWC & being the owner of a similarly upgraded 4,1