Ok, as stated above, the 940 socket AMDs prefer ECC ram. But the 939 Socket AMD Opterons use Non ECC ram. They work in the same socket as standard AMD64 939 processors. The Opterons pass a couple more tests than regular AMD64s. These tests usually are for heat purposes. Servers generally need cooler processors because they like to keep the heatsinks and fans quiet and smaller. So, this is why the Opterons overclock better. Although, some people have also been able to get the same results with some of the 4X00 series AMDs.
The unlocked multiplier is the only difference with the FX's as opposed to the regular AMDs. The multipliers are unlocked for those "Overclocking" people that like to push a processor some more. I'm one of them.
Hope that helps.