I would say it depend on the driver of the graphic card.

windows 98 only does desktop extension, if you need cloning you need vendor software like CCC

Edit: never mind, misunderstood what you wanted to do...

Yes you need the driver to work for this. I had the same "issue" with the Matrox G400, but the driver works of course. In the standard Windows options you can only enable extended desktop.

I'm using the drivers someone linked me in one of my old threads.
If I install Catalyst, the video driver doesn't work at all, if I don't, it does.
I dunno if you remember the struggle on getting 3D acceleration working in my Tomb Raider 3 thread.

How you you capture?

Second PC with capture card?

720x576 or 768x576 is Pal-Standard (in Germany for example)

You can try 640x480@60 at Monitor and set Video Output and Capture Card to NTSC 640x480@30.

And for CCC maybe you need MicrosoftFramework.

If capturing is your main endgoal , I would use a 100hz Vga Splitter, the one I got actually fixes the strange resolutions found in old dos games and allows me to do captures of dos games, without any hassles

My current capture card is an Hauppauge HD PVR, I can connect to it only S-Video, RCA (yellow, white and red cables) and Component (red, green, blue, white and red cables)

With this Trust Televiewer v2 thing, I can convert the signal from VGA to RCA without problems, but it looks very low def and blurry in the capture card (although it looks very crisp and nice on the monitor with the Televiewer's VGA output iself).

Faced same issue, but wanna go deeper, when I play on single screen, my dos games have high resolutions, but if I go with dual screen mode, I’m getting only tiny resolutions.