What 6 core CPU to buy

Hi guy i have mac pro 2009 quad core 2.66 single core. Looking to upgrade to 6 core. i did bit o research and before i pull the trigger can you tell me which one to get and if they will work. I mean hey all have same spec 6 core 3.33 one is 3.46 what is the difference here?

I also see 12 cores intel Intel Xeon E5 2670 V3can they it in to my mac?

Thanks guys

Intel Xeon X5680 3.33GHZ 12MB 6.4GT/s LGA 1366 Hex 6-Core CPU SLBV5

Intel...

What 6 core CPU to buy