The Mac Pro (Early 2008) can support 64 GB of RAM but for best performance, you need to set boot-args maxmem=63488 which reduces the amount to 62 GB. This works for macOS versions starting with Lion 10.7.5. There is a similar setting in the BCD for Windows. I believe Linux does this fix automatically.
For Leopard 10.5.8 (which supports only 32-bit kernel), I had to set maxmem=32768 (I did not try higher values except 63488). For Snow Leopard 10.6.8, I guess the same is required when using the default 32-bit kernel, or you can do arch=x86_64 maxmem=63488 to use the alternate 64-bit kernel with 62 GB or RAM. Later macOS versions only have 64-bit kernel.
Instead of setting the values in NVRAM, you can add them to the com.apple.Boot.plist file so each OS can have their own values.
If maxmem is not set to below 64 GB, then performance is much slower in macOS and Windows (disk activity slowdown?). The CPU-Z benchmark can show the problem in Windows. A macOS installer may take hours instead of minutes. I think OWC needs to do more testing of this 64 GB configuration. I've tested all macOS versions from Leopard 10.7.5 to El Capitan 10.11.6 (and Sierra 10.12.6 to Catalina 10.15.6 with install patchers). There's probably an Intel CPU manual that explains this problem.