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If you go with Dell, pay attention to the power supply. I've priced a few Dell machines over the years, and some have seemed like great deals, but the power supply was like 350W. So you can upgrade some parts to make it more of a gaming machine...but the PSU isn't scaling with the other parts.

I called Dell once to ask about the option to select a different PSU, and they said "we can't upgrade it, there is only one option." Not sure if that's still the case, just something to be aware of.

the weakest point here is RAM, yes, so you'll have to change Motherboard, RAM and processor.

As far as VR goes another point to consider is your GPU, a 970 was the minimum requirement and that was at launch. As devs start getting a better idea of what they can do and start pushing out more demanding games cards on the minimum end of the requirements may struggle to hold up. I don't know how much of an issue this actually is at present but its definitely something to consider as I've seen quite a few posts in places where people have suggested something more powerful than than a 970 for VR systems.

Depends on the platform and overall setup. Ryzen and non-dedicated GPUs (iGPUs / APUs) can most certainly be affected by RAM speed. Intel CPUs are much less affected, and in general if you look up benchmarks you'll see the average framerate differences are negligible, but that's only counting average and max framerates. Framerate consistency wont necessarily be reflected by average/max framerate statistics, the same way that micro stutter (often found on SLI setups) wont necessarily be.

As for sources, that's tough because this comes from my own firsthand experience. Even if I dug around to find benchmarks using older RAM and a newer GPU, performance variances could be dismissed as being the fault of the CPU. OP is in an unusual spot where he wants to game using an older multi-CPU workstation with slow ram but a fast GPU. I don't think I can find online framrate timings with modern games using a system comparable to his. But I do believe I am correct in that RAM that's about 1/4 the speed found in a modern system will have an impact.

so basically - his case is 9 years old and he upgraded almost everything important =))

Yeah, I figured that out =)

Not really, no. Also not worth fiddling with extra drivers and increased fault risk.

Nah, a fast NVMe SSD will be almost always bottlenecked by the CPU.

Multipliers are usually CPU locked either-way, you need to up the baseclock/fsb, but server parts aren't supporting that often.

crazy customization of the system options and expandability

like what? Except the dual-CPUs it seems worse all around compared to a normal pc. I got my 8 year old i7-920 from 2.7 to 3.61ghz, a 30+% improvement which made it still suitable for near everything.

Are you requiring those 2 CPUs for anything demanding like rendering?

or go raid 1+0 and lose money on getting redundancy =)