This image should be of some help:
https://vgamuseum.ru/wp-content/gallery/tride … ui9400cxi_f.jpg

These are 40p SOJ sockets and in this case you want 4Mbit (512kB) (FP-)DRAM chips with 256k x 16 layout. The speed must me at least the same as your existing chips. Your photograph isn't sharp enough to read it, but I'd assume the 70ns chips used on the card in my link should be fine. So look for 70ns or lower 256k x 16 4Mbit FP DRAM SOJ-40 chips and you should be good.

But...

I'm not sure you'll get all the benefit you might expect from them. I can't find a datasheet from the TGUI9400CXi, but there is this (very high-level, dare I say "marketing") datasheet for the TGUI9440 which references the TGUI9400CXi and basically says it's the same chip but with more GUI acceleration features included. So the "24-bit TrueColor RAMDAC" - which determines available modes - should be the same. It then goes on to list its "Enhanced Display Capability" including: "1280x1024-256, 1024x768-256, 1024x768-32k, 800x600-64k or 640x480-16M colors".

Now, with 1MB you can already do 1024x768-256, 800x600-64k or 640x480-16M. All you are gaining from upgrading is the ability to do 1280x1024-256 (don't bother, output quality at this resolution will be too poor) and 1024x768-32k, which is interesting, but not well supported. What you don't get are the main reasons for wanting 2MB, either 800x600-16M or most importantly 1024x768-64k, the resolution/colour combo that people would have wanted to upgrade for. This is a limitation of the DAC, so not one that more RAM or other modifications would be able to resolve.

as @dionb said - don't bother.

Even the superior TGUI9440 VLB won't allow higher resolutions in Windows 3.1 with the full 2mb - I tried. Only the TGUI9440AGi VLB allows them, and you can't use the TGUI9440AGi variant's drivers with the TGUI9440. I don't know about the TGUI9440 variants with the numeric suffixes (e.g. "TGUI9440-1", "TGUI9440-2", etc...) -- I'm not sure what they're all about. So for practical purposes, anything above 1MB with these cards goes completely unused because of the limitations of the VGA IC.

Thanks for the comments here.

My goal is not achieving anything fancy, just filling in those sockets as they were "meant to be". Let's think of it as "maximizing" for the sake of it.

Anyway, here are some better pics, last night I've just cropped the bay photo from where I bought it, although, the forum resizes them automatically anyway.

Thanks guys