I'm genuinely curious, why is it your number one feature? I know that ECC provides error correction, but aside from server use, is it really that critical?
If you've decided upon ECC as a must, then it would be great for a system builder tool to then show you options based upon that choice.
I could see a big data or HPC developer wanting to have a desktop that used the same processor as the cluster they are writing for - mainly in the case where your tuning for max performance and want to able to develop using the same complier flags.
http://cr.yp.to/hardware/ecc.html
He hasn't updated is build in a while, though. http://cr.yp.to/hardware/build-20090123.html