It was back in AMD socket A and socet 7 days, but it may still work. At that time and probably still now, the SPD was always read off the DIMM1 board and that set the speed for the entire RAM, if you set it that way in the BIOS. That meant slow chip always went first if you had speed mixed chips, so both would run at the same slower speed. I discovered however when I wanted to run windows 98 with 1.5 GB of RAM, that the ECC RAM was the only one that came in a 1GB size for DRAM. I was able however, by using the standard 64 bit RAM in the DIMM1 and putting a 1GB 72 bit RAM in DIMM2, the motherboard treated both as if 64 bit RAM and it worked just fine.
I've a bunch of 8GB PC3-12800R (1600MHz) DDR3 ECC REG server RAM modules and would like to find out if anyone knows specific desktop motherboards these would work in. In the past I've used ECC memory in off the shelf motherboards, but that was a long time ago.