I just built a dual opteron system. Initially I ordered 1gb of ECC Registered, however it was kinda pricy. I wanted another gig, but I ordered 2x Registered but no ECC. They work fine, you would have to disable ECC for all banks tho. All opterons need is Registered, ECC is not required. If you're a server and want to make sure that 1 bit in a trillion that gets flipped because of some stray cosmic ray, then get ECC. Or you are doing lots of math stuff, cause that one flipped bit would screw it all up.