The Micronics W6-Li 440FX-based dual socket 8 motherboard that I ran years ago:

FPM and EDO DRAM memory […]
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FPM and EDO DRAM memory

ECC support via chipset when using parity DIMMs

70ns or better

8MB (1MBx64/72)
16MB (2MBx64/72)
32MB (4MBx64/72)
64MB (8MBx64/72)
128MB (16MBx64/72)

There are three (3) types of DIMM wiring configurations: Non-parity - 64 bits wide, Parity and Error Checking and Correction - 72-bits wide. All three types of DIMM configurations may be used on the W6-LI, but not simultaneously.

Depends on your board, my 440fx board I thought supported buffered ram until I picked up 1 stick and found out the notch in the ram didn't line up. =/

I think I have to check the memory slots and see what they accept.

And have a look at the standards for different Dimm memories

This is the difference I see between the 1 stick of buffered edo I have and the other sticks of unbuffered.

I dont know if this covers your machine but on wayback machine I found a listing from MemoryX under Dell OptiPlex GXpro

128MB Micron/Dell Optiplex 168 Pin EDO ECC SDRAM DIMM

311-0189, KTD-WS400/128
Micron is a world leading manufacturer of memory modules.
128MB EDO unbuffered 4K 3.3v ECC Dimm
The 311-0189 carries a MemoryX lifetime exchange or repair warranty against manufacturing defects. It may also be returned in original condition within 14 days of delivery for credit.
Manufactured by Micron
New, Lifetime warranty
Compatible with Dell PowerEdge OptiPlex GXpro 6180, OptiPlex GXpro 6200.

That is the same machine I have.

So it seems like it has to be unbuffered memory in it.