OVERKILL
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I thought you could do at least 48gb on a single, maybe 64gb. You can use the same 16gb DIMMs I have in mine. There's some discussion over using more than three DIMMs of that size per CPU being detrimental, which is why I stopped at 96 even though I was deep enough into at that point that it wouldn't have been a huge deal to go to 128gb.
I haven't used Parallels aside from a little bit of time tinkering with/adjusting other set-ups(like the Trashcan I mentioned above). I gravitate toward VMWare since it's a lot more versatile than Parallels. At least per my understanding, Parallels is basically a Windows-only program. My VMWare library currently has functional installs of Snow Leopard, Windows XP, and Windows 7.
At one time, I kept a CentOS VM going to run specific piece of software. That particular software(vNMRj) is meant for RHEL, but since CentOS use to effectively be RHEL without support, it worked without too much effort. Two things happened to change that. The first is that CentOS quit free RHEL and instead in a weird spot of being kind of the "stable" release of Fedora and not the all-kinks-worked-out rock stable RHEL, The second thing is that when Agilent dumped their NMR division, they released the source code to vNMRj, a bunch of former Varian/Agilent engineers started maintaining it as FOOS, and also started releasing a version nicely pre-compiled for macOS. I'm not sure if the macOS version could actually be plugged in and operate an NMR(you still need a bunch of underlying stuff that's not open source to talk to the spectrometer and I'm guessing that's probably still RHEL-only) but it's perfect for all I ever used it for, which was offline dataprocessing.
All of that aside, I'm super happy with VMWare. It's not as "slick" or easy to use as Parallels, but is miles ahead of Virtualbox in ease of use and, like I said, I like the versatility.
Yeah, I used to run VMWare on RHEL and even Fedora (home system). I've not tried an OS beyond Windows in Parallels, so you bring up a good point on potential versatility, I hadn't thought of that when tabling the question.
RHEL is "free" for small business/personal use now eh? There was a recent announcement. I maintain a few RHEL servers, so I'm always getting their newsletters. It's a great server OS, but if I replace MacOS with Linux on this box I'd probably go Fedora.
On the RAM thing, I was just going by Apple's specs:
Mac Pro (Mid 2010) - Technical Specifications - Apple Support
support.apple.com
If I can do 16's? If that's the case, that might extend the life of this old rig even further, lol.