Does it work properly at 800x600 (or above) if S-Video is disconnected?

Yes, the VGA does display properly when not using the S-Video Output.

Does the S-Video shows correct on a TV?
Maybe the capture card just can't handle some video timing of the Voodoo3 set to 800x600 ?

The S-Video does display correctly on my Elgato Game Capture HD as I've mentioned.

The options that come to mind are to use S-Video splitters and output one to another display source and the other to my capture device (I could use the Elgato capture source for my display, except that the problem is that the capture delay is horrendous.), or there's probably specific monitors that work sufficiently when using S-Video and VGA at 800x600 at the same time? Or maybe specific drivers?

Why can't you use a VGA splitter?

While I did order several VGA splitters and am still waiting for them to arrive from China, I don't think it would make any difference at all when using both VGA and S-Video at the same time at 800x600 resolution.

If anything, unless there's an alternative solution, it may work best if I just use the S-Video Output alone for display and to capture footage using my Elgato Game Capture HD.

And I've also tested it for DOS games using both VGA and S-Video displays and wouldn't even display at all on both ends considering that they usually use 320x200 resolution. So, my conclusion is that as far as using both VGA and TV displays, aside from 800x600 looking a bit off (and sometimes like garbage), I'm extremely limited to that resolution in addition to 640x480. But when using S-Video (or VGA) only, everything works as it should be.

Funny, to me it happens the other way around - 320x200 looking unplayable through V3's S-Video for certain games

A lot of graphics cards display differently through VGA when you use clone mode with S-Video.

What I used in the past is a VGA to S-Video adapter which has a VGA pass-through. That worked quite well.

The other option is to capture VGA with something like an AVerMedia Gamebroadcaster HD like in this video:

https://www.youtube.com/watch?v=vijnjjTka4c

Mau1wurf1977 wrote:

A lot of graphics cards display differently through VGA when you use clone mode with S-Video. […]
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A lot of graphics cards display differently through VGA when you use clone mode with S-Video.

What I used in the past is a VGA to S-Video adapter which has a VGA pass-through. That worked quite well.

The other option is to capture VGA with something like an AVerMedia Gamebroadcaster HD like in this video:

https://www.youtube.com/watch?v=vijnjjTka4c

I've been actually looking into that. The problem is I don't have a working desktop anymore, considering that my current one is processor-less. Plus, it's probably outdated by now. So, I'm basically stuck with an external capture source at least until I get a spanking new powerhouse desktop, which'll take upwards to a lot of time and money. Maybe if the budget allows, I may get it just to have it in case I get a super decent desktop PC once again.

Though on the other hand, what I am using right now is a VGA to RCA converter box, which is an AverMedia 300, and while the quality is a bit degrading compared to plugging it directly into the V3, it does the job regardless. Can't complain really. 😀

Also, if you can, try to get the V3's desktop res in a custom 720x480 mode. It's got the least distortions that way.