Review this sites info: [http://www.everymac.com/systems/apple/mac_pro/index-macpro.html|EveryMac - Mac Pro's]. Locate your exact system and post an update to your question with the info as there are a few different models and not all of them can be upgraded.
=== Update (03/23/2016) ===
The MacPro5,1 uses two 6 core Xeon CPU's: [http://ark.intel.com/products/47920/Intel-Xeon-Processor-X5670-12M-Cache-2_93-GHz-6_40-GTs-Intel-QPI?q=Xeon%20X5670|X5670
(12M Cache, 2.93 GHz] or [http://ark.intel.com/products/52577/Intel-Xeon-Processor-X5675-12M-Cache-3_06-GHz-6_40-GTs-Intel-QPI?q=Xeon%20X5675|X5675
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(12M Cache, 3.06 GHz]. But without knowing which logic board you have as you didn't tell us what exact model you have I can't tell you if these CPUs will work or not as the sockets and the firmware within your system will dictate if these CPU's will work.
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(12M Cache, 3.06 GHz].
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+
But without knowing which logic board you currently have as you didn't tell us. I can't tell you if these CPUs will work or not as the socket and the firmware within your system will dictate if these CPU's will work.
Review this sites info: [http://www.everymac.com/systems/apple/mac_pro/index-macpro.html|EveryMac - Mac Pro's]. Locate your exact system and post an update to your question with the info as there are a few different models and not all of them can be upgraded.
+
+
=== Update (03/23/2016) ===
+
+
The MacPro5,1 uses two 6 core Xeon CPU's: [http://ark.intel.com/products/47920/Intel-Xeon-Processor-X5670-12M-Cache-2_93-GHz-6_40-GTs-Intel-QPI?q=Xeon%20X5670|X5670
+
(12M Cache, 2.93 GHz] or [http://ark.intel.com/products/52577/Intel-Xeon-Processor-X5675-12M-Cache-3_06-GHz-6_40-GTs-Intel-QPI?q=Xeon%20X5675|X5675
+
(12M Cache, 3.06 GHz]. But without knowing which logic board you have as you didn't tell us what exact model you have I can't tell you if these CPUs will work or not as the sockets and the firmware within your system will dictate if these CPU's will work.
Review this sites info: [http://www.everymac.com/systems/apple/mac_pro/index-macpro.html|EveryMac - Mac Pro's]. Locate your exact system and post an update to your question with the info as there are a few different models and not all of them can be upgraded.