^ Well if you are investing so much money on that workstation, then better to go with reliable and higher quality parts...So my vote goes to Xeon and ECC RAMs...
The Xeons are better binned than the i7s and ECCs as you know is the way to go if you have more RAM that too in a server/ workstation environment...
I'm only planning on getting 1 processor
single slot xeon processor --> xeon 3580
supports only unbuffered memory
for some reason ECC, unbuffered memory is 3 times more expensive than ecc buffered or non-ecc unbuffered.
can you take a motherboard that has 2 sockets and only use 1?
that would let me use a single 5000 series xeon with ecc, registered ram