I own a Gigabyte P55-UD3R, paired with a Core i7 860. For many years, it was the core of my home workstation, with 4GB then 8GB central memory. Now it is in my residential server. But with NVME SSDs, virtualized machines and multiple ZFS pools, I wanted to put all the RAM I could on this old motherboard.
The user manual says it can handle up to 16GB, but at this time the was no DDR3 8GB sticks available on the market.
The BIOS has its RAM limitation, the P55 chipset has its RAM limitation, and the CPU has its RAM limitation.
After searching the Internet for answers, I ended to forums threads like this one, this one, or this one.
Finally, I crossed my fingers and bought four of these 8GB DDR3 modules (affiliate link), and... a year later, I can say that it's working perfectly!