Upgrade mid-2010 mac pro (Tower)

There are three types of SSD that can be fitted to the classic Mac Pro and one of these does not support booting from.

1. A SATA 2.5" SSD like a Samsung 850 EVO or 860 EVO
2. A PCI M2 AHCI SSD like a Samsung SM 951 AHCI SSD
3. A PCI M2 NVMe SSD like a Samsung 950 Pro or 960 Pro SSD

The last of these i.e. an NVMe SSD cannot be booted from.

The first of these i.e. a SATA SSD as of itself would come as a SATA III model which in theory can reach 600MBps speed, in reality even it would be nearer 450MBps. Unfortunately however the classic Mac Pro only has SATA II connections which max out at a theoretical 300MBps and this gives in reality a speed of only 250MBps even for one of these SATA III SSD drives. You can get up to 2TB capacity SATA SSD drives. If rather than fitting the SATA SSD to one of the built-in drive bays you instead get a PCI to SATA adapter like the Sonnet Tempo adapter mentioned by kahjot or one of these https://www.apricorn.com/upgrades/vel-solox2 then it will be able to run at SATA III speed.

A PCI SSD can reach approaching 1500MBps which is clearly much, much faster. (This applies to both AHCI and NVMe types.) However as mentioned only AHCI ones will be bootable. You will need a PCI adapter card like this https://www.amazon.com/Lycom-DT-120-PCIe-Adapter-Support/dp/B00MYCQP38

AHCI SSD drives are becoming harder to find, they are older, slower technology. Samsung for example seem to have discontinued theirs. Even if you find one I suspect it will be limited to a maximum of 1TB capacity. You might find a 2TB NVMe one but as mentioned it cannot be booted from in a classic Mac Pro.

For a video card upgrade logically you would want one that supports using Mojave aka macOS 10.14. However unless you find an old secondhand AMD Radeon HD 7950 (Mac Edition) or old secondhand Nvidia GTX-680 (Mac Edition) it will not support the pre-boot option screens and hence not support FileVault. See this article for a list of video cards that would work with Mojave which includes the two Mac Edition cards I mentioned.

See - Install macOS 10.14 Mojave on Mac Pro (Mid 2010) and Mac Pro (Mid 2012) - Apple Support

Crucial SSDs have been solid for me. In terms of a speed boost, you might want to get a card to hold the SSD.

Tempo SSD PCIe Card Adapter | Sonnet

There are other SSD adapters, some of which use m.2 SSDs.

It will be a bit below 1500MBps that is the theoretical speed, real world will always be a bit slower. As a real example I have a genuine Apple SSD which uses a slightly different type of PCI card but would give similar speeds, mine gets over 1400MBps.

I also found from prior research that the bigger the SSD the faster it was so a 1TB is faster than a 512GB which is in turn faster than a 256GB. Remember you are likely to only be able to find a max of 1TB for AHCI drives.

The following article is not exactly the same config but the figures are inline with what you can get on a classic Mac Pro with the sort of SSD we are discussing.

See - http://barefeats.com/hard218.html