Just sell it if you dont want using it your own.

1. The weakest a 486DX4-100 VLB - with Cirrus (5428 I think) graphics (alternative card available Diamond Stealth 64 Video VRAM) […]
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1. The weakest a 486DX4-100 VLB - with Cirrus (5428 I think) graphics (alternative card available Diamond Stealth 64 Video VRAM)
In it's previous life, a local computer club machine with - I think it was win98 - haven't run it for ages.
Guess it might make a passable DOS machine, though too fast for speed sensitive.
Would make a great Win3 machine though.

Too fast for Win 3.x. If it has 98 it surely has quite a bit of RAM.
I would go with 95.

5. Couple of P4-640 machines, but horrendously unreliable, and one is a friend's old one (same model) that has already been clean installed, so it doesn't feel like software rot - kind of ignoring these as I have very little trust in them.

Broken P4s? 🤣
Salvage any decent parts and junk the rest.

Any of the older things and all the cards would be sellable to collectors

There are two types of things that I always keep:
- any 486 and older hardware
- anything with the name 3dfx

I also keep nice parts - like your Matrox card for example.

Considering your pile, what you do with it depends on what you want. If you wish to go retro, it's always nice to have a good pile of hardware for testing purposes. That said, I would keep the working stuff and ditch all the rest. But I would definitely ditch MX, I hate these cards.

The 4MX is my best AGP card - Though it seems that Geforce 4 4200's turn up for not too much money - for a classic DX8 card - which would be an advance on the MX (a glorified Geforce 2).
The Matrox, surprisingly, was a computer fair cheap bin pick - looked and thought hey, that's a big name.

I'm leaning toward the "best of the rest" build, so that would probably get the SB Live, and maybe a better AGP card - target - run things I wouldn't risk polluting Windows 7. Win98 in the mix would also be a backdoor into "real DOS".

I really ought to soak test the P4s, maybe between them there might be one good one

I like the Geforce 4 MX, Geforce 2 Ultra with a different name.

When it comes to the P4 machines, look for bad caps on the motherboards and throw away those boards which are bad and keep the rest (if any).
I would keep all the CPUs, memory and drives in any case.

If the PSUs and cases are standard ATX or mini ATX I would keep them, if they are HP/Compaq low profile garbage I would trash them if the motherboards are bad.

Yeah keep the 486 and the IBM PC330 or sell it but don't junk it. As for the rest meh, nothing special you can still find most of those computers at garage sales and such.