I tried with an A4000 and an internal scandoubler plugged into the video slot. Didn't work well at all. I mean, I got a picture, but it was all squashed up into the top 50% of the screen and no amount of tweaking the monitor controls would fix it. That was PAL 640x256. Same happened with 640x512 interlaced.
An alternative might be to purchase a TFT monitor that takes a regular video input, like the Samsung ones. Some now have S-Video and Composite video inputs and this should display the AGA screens well.
