Thank you all for your advice. Just looking for a good upgrade to the boot drive to give me a good performance hike to the standard HDD I have.
be it a Samsung Evo SSD. Which interface would give you the best performance?
Best performance is from m.2 NVMe SSDs ~3,000MB/s (but limited to ~1500MB/s on classic Mac Pro due to PCIe v2 x4 bandwidth) then m.2 AHCI SSDs such as the SM951 AHCI ~1500MB/s (but these are older and harder to find these days) then your standard run-of-the-mill 2.5" form factor SATA AHCI SSDS ~600MB/s. Samsung Evo can refer to either an m.2 NVMe SSD or a SATA AHCI SSD but I assume that you mean something like a 970 Evo or 980 Evo both of which are m.2 NVMe SSDs.
To boot from an NVMe SSD you must upgrade your machine to Mojave first to get the latest 144.0.0.0.0 firmware. In order to do that you need a Metal supported GPU which it sounds like you now have (RX 580.) Samsung 970 Evo is an excellent choice. Samsung 980 Evo works too but will be a waste unless you plan to use it on another computer someday. A WD Blue M.2 is also a good choice, WD Black M.2 also works, but like the 980 will not reach its full potential in this machine. Sandisk and other manufacturers also work if you find something cheaper. Watch out for server-grade SSDs (e.g. HP Exo) as they tend to have slower write performance.
If you'd rather get a standard 2.5" AHCI SSD for the lower price and good enough performance just about anything will do as they are all practically identical in performance at this point. I just threw a Leven (non brand name) 2TB SSD in a Mac mini and it performs just as well as the (slightly) more expensive ones from WD, Samsung, and Sandisk.