You can easily understand, then. I built my last PC 4 years ago. It was a great Intel chip, and there was mild criticism in the forums because I upgraded it to the 14nm refresh "toc" chip -- the Kaby Lake. Everybody was trying to tell me, or insinuate perhaps, that I needed to build a new i9-10900K or even an i7-10600K deca-core or hexa-core. Of course, all these new chips use Skylake cores, new and improved times 6 or 10.
I developed this "formula" for building a tower desktop four years ago with that quad-core chip. For all I did with what I had, all the refinements, the fan deployment, the Lexan duct I built for the motherboard and the Coolermaster Cross-Flow barrel fan as a specialized exhaust, I was so happy with it that I decided to repeat the project this year with the spare parts acquired just because I needed to replace my motherboard (blowing out the USB controller with a *$%#!! vaping-pen and my body's static charge), and started spending "stimulus" when all would've been well for just $88 shipping to ASUS.
Now I'm looking for an LG BD-RE optical reader-burner. It took me a while! I keep thinking I'm getting behind this technology and that I won't find the parts I want and therefore "need". I think I found my answer in the LG BU40N BD-RE, 9.5mm thick. I've been using laptop parts in my desktop PC builds.
Anything I did and plan to do with the case, drive-deployments, hot-swap bay with ODD slot -- can be quickly converted with an upgrade to these fancy new CPUs. I just don't feel the need for six cores!
There is no bottleneck for older NVME drives like the 960 Pro/EVO or even the 970 Pro/EVO, if the PCIE bus is version 3.0. I don't know how much version 2.0 would cripple the throughput, but it shouldn't be hard to find out.
Motherboards began appearing with PCIE v. 3.0 around the time of the boards released for both Sandy Bridge and Ivy Bridge, but you'd only get PCIE 3.0 if you used the Ivy Bridge chip. That's all I can say from my personal experience. To get the full spec of performance, you need a slot providing PCIE x4. And again, it should be easy to find out what PCIE v.2.0 poses for performance limitations. I still imagine that it would be vastly better than the top performance of any SATA HDD or even SSD.
I ran a search for the "Dell 790" and I turn up the Dell Optiplex 790, and the model that appeared on my screen has a sandy bridge i5-2400 in it. So you likely have PCIE v.2.0 in the hardware. I wouldn't be deterred, though; I'd just try to determine what I have, what I would get with NVME, and then make my decision as to whether I would be happy with it. I really think if one would investigate further, it might be worth capping off the experiment with a purchase of a larger capacity NVME. You'll have to find out.
Those 10-year-old Sandy Bridge systems last a long time. I've still got two of them running, and they don't miss a lick -- one of them being about 11 years old. Running 24/7/365, too.
My dentist buys corporate-asset-turnover Dell refurbs. He says he just replaces the PSU in them, and he's perfectly happy.