just buy tried and true chips
Unless you are getting a huge bargain on those chips, the X5670, X5680, or X5690 are tried and true chips on the 4,1 and 5,1 machines. I installed dual x5670s on my 4,1 just by following instructions from Pindelski's website, and it pretty much got me there within two hours (one hour wasted shooing away my 5 and 7 year old children).
Mac Pro (Mid 2010)
Single-Core Score Multi-Core Score
Geekbench 3.1.6 Tryout for Mac OS X x86 (32-bit)
2329 23461
Result Information
Upload Date November 14 2014 03:54 AM
Views 1
System Information
Mac Pro (Mid 2010)
Operating System Mac OS X 10.10.1 (Build 14B23)
Model Mac Pro (Mid 2010)
Processor Intel Xeon X5670 @ 2.93 GHz
2 processors, 12 cores, 24 threads
Processor ID GenuineIntel Family 6 Model 44 Stepping 2
L1 Instruction Cache 32 KB x 6
L1 Data Cache 32 KB x 6
L2 Cache 256 KB x 6
L3 Cache 12288 KB
Motherboard Apple Inc. Mac-F221BEC8
BIOS Apple Inc. MP51.88Z.007F.B03.1010071432
Memory 65536 MB 1333 MHz DDR3
Integer Performance
Single-core 2459
Multi-core 26930
AES
Single-core 1864
1.60 GB/sec
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Well, not so great for single core performance, but not too shabby for multicore performance. Really cheap and it works.
Although, I swear that the results were closer to 27000 multicore when I first tested it in Mavericks -- and this Yosemite build is 10.10.1 -- perhaps not ready for primetime?