Can I use non-ECC / non-registered ram in Mac Pro 2009?

The supported ram for my single-socket 2009 machine is apparently registered/ECC. However, I'm curious if I can get away with non-ECC and/or unbuffered, especially if I'm not using the 4th slot and not mixing with other memory types.

I'm also planning to up the firmware to 2010 and drop in a W3580 if that makes any difference.

Thanks!