There is a thread on this from back in 2010. I found out the hardway that the Mac Pro 3,1 8-Core maximum memory is 56GB. That is with 6 slots at 8GB and 2 slots at 4GB. I was previously running 48GB with 4 slots 8Gb and 4 slots GB which also runs extremely well and it is possible that 48GB could be more performant than 56GB because you can pair things in two directions (on the Riser and between Banks A and B of the two risers). The theoretical max on an 8core machine is 64GB. However, with 64GB the performance is unusable and will lead to KP's right after IOConsoleUsers is loaded or after the login screen. Also, your RAM will become an Easy Bake oven.
Reference:
https://forums.macrumors.com/thread...e-48-gb-fast-disks-why.1940030/#post-22338450
Apple lists the max at 32GB. And that was either by design and/or chip availability. In an 8-core Mac Pro 3,1 the you can run 48GB or 56GB without issue. Just be sure to put your RAM in pairs and if you can get banks A and B on the two risers to match, that will be faster as well. Obviously this cannot be done with 56GB RAM, you will end up with 1 pair that does not match the other bank.
That said, 56GB system seems faster to me than 48GB, but I did change the VM setting, so I don't have an Apples to Apples comparison and no speed tests yet between 56 and 48GB.
The thread above also mentions a nvram vm compressor 2 setting that disables Swap. It will still allocate some disk space to the VM volume but the page ins and page outs should be 0.
Example:
VM: 1909G vsize, 1379M framework vsize, 0(0) swapins, 0(0) swapouts.
Somewhere I saw it mention that Virtual Machines will run faster too, but have not confirmed that as I am in the middle of installing fresh systems as I as troubleshooting that the issue was on my box. I did many upgrades in a short amount of time and discovered before even reading about it that 64GB RAM was not good on this box. Mac Pro 5,1 should not this issue.
I should have suspected this sooner but at the time as I did seem system crawl after 64GB RAM was installed. But I thought maybe the system was caching something. Then I thought power, but lack of power or a power overload usually shuts down a machine completely and this was KPing at startup when graphics card kicked in and it happened even on installer disks and other startup volumes that were not touched. At that point, it thought it was my GPU but after a little more troubleshooting and starting with just 32GB, I found it was memory all the long and Worked by way up from 32GB to 56GB.
My main reason for adding RAM is running older Virtual Machines on this box and testing CloneToolX with RAMDisks to speed up testing. It will be maybe twice as fast on this system. And I originally thought it would be cool to max out this box with the original intention of 64GB, but was able to at least get 56GB. So to anyone who was a Mac Pro 3,1 with 8-Cores I would go after 32GB, 48GB, or 56GB and you should be happy!