I had some issues with getting a IIfx to display video on a Dell LCD. I think it has something to do with sync on green. For me, the IIfx decided to wash out the image on the LCD and it would do that with both video cards and different DB-15 to VGA adapters. Using an official DB-15 Apple monitor solved that issue (mind you I have the monitor that outputs VGA but the cable adapts it to DB-15...something to try later). It didn't like my Acer monitor either (but it liked an 19" widescreen I used to have).

What specifically is your IIsi doing (or not doing)?