LOL, yes dual Opterons in SLi with nForce4 is nice!! The scientific difference between non ECC and ECC ram is simply that the ECC ram takes more time, but uses Error Correction Codes (ECC) to provide more stability for the user. This ram is, like already stated, used in servers. Don't bother with it at all.
I have DDR1 on my FX-55 nForce4 MOBO, and it flies! I love it, but be wary of compatibility issues as well.
The trouble is, DDR2 is not yet supported by AMD processors due to their need for low latency ram. So, I guess I oopsed on that prior post, but to tell you the truth, DDR1 and DDR2 are almost the same speeds in reality. DDR2's latency is so high, that DDR1 at lower speeds is just as fast.