Nvidia Secret Sauce for ECC?

http://www.anandtech.com/show/2977/nvidia-s-geforce-gtx-480-and-gtx-470-6-months-late-was-it-worth-the-wait-/4

See http://semipublic.comp-arch.net/wiki/Poor_Man%27s_ECC

In particular
US patent 7,117,421, Transparent error correction code memory system and method,
Danilak,
assigned to Nvidia,
2002.

http://semipublic.comp-arch.net/wiki/Poor_Man%27s_ECC#Optimizing_Poor_Man.27s_ECC