On Gigabyte site these motherboards specs all claim to support ECC RAM but "Use of a CPU that supports ECC is required."
GA-MA770-UD3
GA-MA770-UD3P
GA-MA790FX-DS5
GA-770T-USB3
GA-MA799FX-DQ6
GA-790FXTA-UD5
So I go to each ones 'CPU Support List' and find no mention of any supporting ECC. Where is this info at? Which are the last (most recent) CPUs that support fully buffered ECC?

Fully buffered? I think only Xeons do that, not any of AMD tribe.
Phenom II supports ECC but unbuffered (poor man's server RAM).

Just about all Phenom/Phenom II, Athlon/Athlon II, and FX desktop CPUs support unbuffered ECC RAM. I do not believe Semprons do, and I don't know if the Socket FM1 Fusion APUs support ECC or not.

However, be very careful about buying ECC memory. Desktop CPUs can only support unbuffered ECC memory, they do not support registered ECC or fully buffered (FB-DIMM) ECC memory. Registered memory is only for use in servers, although it will fit but not work in desktop motherboards. Most ECC memory is registered, so you will need to look carefully at the memory before you buy.

FB-DIMMs are DDR2 based and were used in some older Intel and Sun servers. They will not fit in a machine that was not designed for FB-DIMMs. FB-DIMMs also are known to run notoriously hot and REQUIRE a metal heat sink. Most other kinds of ECC memory don't have heatsinks. The ones that do are nearly always registered, so stay away from ECC memory with heatsinks in desktop systems.

If you are in for historical trivia, the Xeon 5200s and 5400s with 1600 MHz FSBs on the i5400 "Seaburg" northbridge were the latest CPUs to support fully buffered memory.

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