Steve
Thanks for the extra details.
You would likely be dead in the water with 8GB RAM. Sounds like you know that and were planning on a upgrade. Everything uses (and can benefit from) more memory than you are used to. 16GB is the minimum but in reality, with disk cache you may want a lot of memory, and if you do use VIs or have been working around memory constraints in Pro Tools 10 then definitely aim for more. Personally I would max out the memory. You'll be limited to 48 GB max memory, look online like on EveryMac and confirm for yourself or call a dealer like OWC.
You have SATA HDD... but no info on make or model, so I can't guess what performance this has, or wether it's a model that has issues or not. I assume it's connected to the motherboard SATA, which is only SATA II. Likely the SSD is SATA III so you lose some performance there (but will still seem fast, especially for non-sequential IO compared to a HDD). The SATA II disk will do a few hundred MB/sec sequentioal I/O, a PCIe NVMe drive is capable of doing a few GB/sec (!) of sequential I/O and you can get these cards in M.2 format for a few hundred dollars. Like this:
https://www.amazon.com/dp/B07C8Y31G1..._SJmiEbZJZF9HE
You need a PCIe slot to M.2 adapter card to install these M.2 cards into a cheesegrater classic.
The issue with the legacy cheesegraters is the PCIe slots are only PCIe 2 so if you put a modern PCIe 3/NVMe M.2 card in a simple adapter in those slots you are running the cards on a half speed bus, real performance is not halved but the loss may be significant--but still so much faster than SATA that even PCIe 2.0 may seem fantastic.
The high-end way to do this is use a PCIe to M.2 adapter card that is switch based and aggregates pairs of PCIe 2 lanes to a PCIe 3 lane. *the* switch based card that people use is the Syba I/O Crest
https://www.amazon.com/dp/B07HYZY7P2..._bMmiEb22PC1VZ
There are great threads on DUC and links to external threads about that card you should find. Read those and be aware of people having problems. The boot time concern some people have seems a little silly to me, and can be optimized by removing legacy devices. Still this is likely where I'd start.
Another interesting card is the Sonnet "4x4" this is really aimed at the new Mac Pro Cheesegrater that has PCIe 3, I believe this card does not do PCIe 2 to PCIe 3 link aggregation (if it did it would require you to give up two of the four M.2 slots).
https://www.sonnettech.com/product/m...pcie-card.html
But why do you want to add storage? For what? Just a second boot partition? Where are your samples and audio sessions today. One thing to consider is if you move to these NVMe drives you can do things like put your session (and samples) on the boot drive, they are that fast... like if you end up getting say just a new NVMe boot drive for macOS Mojave, just put your sessions there, it will run so much faster than any other drives you have.
What *I* would do is...
Max out the DRAM
Install a metal compatible GPU if you don't have one already (lots of threads on DUC and elsewhere)
Upgrade the Mac ROM to to support Mojave if needed (lots of threads about this online, start by checking what version you have now).
Install NVMe SSD (if you need that for the new install/or just want faster performance). I would likely use the Crest adapter card (after careful review of threads on DUC and links in them)
Do a full installer (e.g. boot off USB flash stick) install of macOS Mojave
Start installing and testing Pro Tools 2019.12 or later... (be aware of video limitations if you work with video).
And maybe do this now: obtain a copy of the Mojave installer. See if you can do that now in the Mac Store, download the full installer (~8GB or so from memory, not the much smaller mini installer). If all you can download is Catalina you'll need to contact Apple support and ask for a favor since your mac can't run Catalina. And as it stands now you will not be able to upgrade your Mac beyond Mojave... who knows if somebody will hack a way to do this in future. So that might affect how much money you want to spend-on this.
Be aware that a modern macOS install will use APFS partitions and when you boot from your old OS X install you will not be able to read those drives. APFS is *great*. The new macOS will be able to read all your old HFS+ drives. Just wanted you to be aware of this, you should be able to easily work around this.
And finally I don't have a legacy Cheesegrater anymore, and I'm a geek who would hack his to do all the above. That may not make sense for you to do. And I don't want you just listening to me. I just wanted to give you a flavor of what's possible and let you go and do more of your own research. If you get stuck ask questions, especially on the relevant threads on DUC. And if you do end up planning to do a lot of changes... it might help to those down in a bullet list of things you are planning to do in order, with details of product make/models and post here and I suspect you'll get experts willing to look over them.
And overall advice... always have a backup/exit plan. Make sure your backup disks are physically unplugged/removed from the Mac Pro while you make changes so you don't accidentally trash them. Plan to make one change at a time, and as much as possible test stuff works at each step. Start on the simpler stuff, like say adding RAM, then a GPU, then flashing the ROM if needed, etc.. And if you really don't need/want to do any of these upgrades... don't. Work out what you really want to end up with. (Again for *me* that would be Mojave with Pro Tools 2019.12 ... and some form of NVMe SSD... but that traps you into lots of upgrades).