Tseng ET4000AX i bet.
idk but thats the fastest isa card.
windows use is meh,

ATI Mach64 VRAM 2 or 4MB ISA

For DOS / VESA graphic modes (Doom, Quake, PCPBench, etc.) i got the same framerates with 486 Dx2 66 with following ISA cards:

S3 801
ATI Mach32
ET4000AX

I would assume, that the S3 928 ISA and ATI Mach64 ISA would perform equal in DOS to the cards above. Also some Cirrus Logic cards should be on par. Please consider, that you need to load VESA drivers for the SVGA modes for the most S3 ISA cards and the Mach32. Some games have own VESA drivers for ATI and S3. For some DOS applications could be the compatibility to IBM 8514 mode relevant (Mach32).

Made no tests in Win3.x but would guess that the Mach64 2MB+ VRAM and the S3 928 2MB+ would perform on top level for ISA cards. Also the Mach32 with 2MB VRAM should be not far behind. The ET4000AX is in Windows the slowest of all the cards above. You should consider the speed of the RAM DAC if you want drive higher resolutions and higher color depths. All in all the best ISA cards for DOS and Windows are S3 928, ATI Mach32 and ATI Mach64, all with 2MB+ VRAM.

There are many similar threads about this topic.

Overall best ISA cards for:

DOS & WINDOWS:
Cards based on Cirrus Logic GD-5434 chipset with 2MB of memory - Diamond Speedstar 64, STB Nitro 64

Best for Windows:
ATI Mach64 VRAM with 2 or 4MB of memory

Best for DOS:
Cirrus Logic GD-5434 but usually very close are other Cirrus Logic chipsets and other better known chipsets from Tseng, WD, S3 ...

There are also a lot of overrated cards.

If you don`t need Windows 3.x/9x GUI Acceleration you can also take Trident 8900 with 32bit Memory.
They usually as fast as their ET4000 counterpart.

The Biggest Limitation is not GPU Speed, more limiting is how fast your CPU can move Data to Video RAM.

A Mach64 Chip would never see full performance with only 8Mhz ISA Clock for example.
Many Boards allow to increase ISA Clock. So you can run them on 10, 12,16Mhz... Bus Speed if all of your cards are happy with that.

I find it amusing to see all graphics cards referred to as "GPU".
This may be true with modern graphics adapters, but in the ISA era vast majority of cards were either dumb framebuffers, or fixed-function accelerators - in both cases no processors there!

About the only things deserving the term "GPU" back then were TIGA graphics coprocessors, and I would say these were the most advanced ISA graphics adapters.
Though they were designed primarily for CAD software, not good as general-purpose cards...

I suppose a real 486DX not to mention @ 66Mhz or similar would push these cards where instead on a early 486 or a late 386 most of them would be almost similar in dos with some difference in windows. I'd go for the "best for its price", I mean like a Cirrus Logic while the GD5434 may be expensive, also the GD5426-5428-5429 should be good in both DOS and Win having internal graphic acceleration for the last and they are not that cheap too. Many version of these chip were built on VLB cards too so for late 486 generations.
ATi Mach64 ones I suppose are awesome but their price and rarity became impossible. At that point I'd have doubts to use them at all from a collecting point of view.
Lately I was discussing about a card I almost forgot the Matrox Impression in ISA version. How good was that? I suppose it had some 3D functions for CAD but considering how good were the later PCI versions in VGA output quality are there any review of this card? I've seen them in different pcb layout online also with "modern" ram modules..

In my opinion, the words "GUI" and "ISA" can only exist in a sentence when choosing between them.
For usability I'd go for at least VLB or PCI (or something EISA? Opti bus? Never used them).
On the other hand, the best ISA cards with GUI support I used are the S3 928 and cirrus 542x. So about the best cards among them I'm pretty inexperienced.

Those cards by the way just take the ISA bus to the max. These cards I tested and performance is similar in dos benchmarks:
- V7 Vega Cirrul Logic, 1MB
- S3 928 1MB
- ET4000AX , 1MB
- UMC Svga 1MB (with TSR)
- OTI087 (just slightly behind).
Tested at ISA bus @10MHz.

Ok after a bit more research I think I've found that the cl-5420 might be more than enough for what i need and doesn't cost very much.

It more or less just needs to display another monitor and play back some video in windows media player. I watched a youtube video of someone playing starcraft and navigating the windows UI in it and it seems like it can do that.

I am not sure but I suppose the GD5420 might not have the BitBLT acceleration for the Windows GUI, if my memory is correct but I'd suggest to be sure about this and considering that if Windows is also a priority to run as fast as possible (compared to the vga price obviously). Still, I might remember wrong but I think the acceleration came later.

What do you mean by "secondary"? Do you plan on using two VGA videocards to drive two screens? I'm not sure it's that easy.

From what I understand, each videocard has its own VGA BIOS and they are likely to conflict. If anything, only one of them will get initialised. There are cards that can be jumpered to be secondary display adapters, but they are either EGA/MDA/Hercules cards or some very niche products.

You can also build systems with cards that use different 3D graphics API and use game settings to choose either of them (e.g. GeForce for Direct3D or OpenGL, Voodoo for Glide), but you're not using them simultaneously.

If anything, your best bet is a single card with dual output — those were made by Matrox and NVIDIA among others, but they are for PCI/AGP.

ISA GPU

That just sounds... wrong.

jheronimus wrote on 2020-07-30, 11:17:

What do you mean by "secondary"? Do you plan on using two VGA videocards to drive two screens? I'm not sure it's that easy.

From what I understand, each videocard has its own VGA BIOS and they are likely to conflict. If anything, only one of them will get initialised.

the machine is isa only and this is within/for windows ui , where you can have as many gpus as desired

In this case you should make sure, that both graphic cards use different ressources (IRQ/adress). So you should chose a graphics card, where you can switch/disable the ressources.

Somehow I can't believe in two ISA VGAs working together.
But feel free to prove me wrong...

Yeah they won't. There is no way for the vram to not occupy the A segment

What you can do however, is use two ISA graphics cards at the same time provided one of them does not have a VGA core or it can be disabled. The original ATi 8514/Ultra for example was an accelerator only, and you could chain it up to another VGA card using the feature connector.
I have an EISA card (ISA version exists too) called Supermac Spectrum/24, which is a 3MB 24-bit card that I chain to an ET4000 for VGA using the feature connector.
On several models of ELSA cards which use S3 chips, I have seen jumpers for setting the BIOS to primary/secondary, as well as VGA ENABLE/DISABLE. So presumably you can use these as dedicated accelerators as well.

AFAIK, OS support by Microsoft for multiple monitors driven by different cards started with Windows 98 and requires PCI or AGP cards .

Before that, it's foggy for me . I seem to recall multiple monitor under Windows being possible with specific vendor hardware and software . I do not remember any specifics, however.

Indeed. You can select primary/secondary VGA card with this ELSA card. But I'm not sure how this works. Maybe I should try it sometime 🤣