i've installed AmigaSys (
http://amigasys.extra.hu/ ), running Workbench 3.9. this package comes with RTG drivers (those UAE .... in ScreenMode Chooser). on a plain workbench install (checked workbench 2.1) there are only native PAL / NTSC. those may not work. so you need to install AmigaSYS, or get the rtg driver - picasso96 from aminet which supports winuae's "uaegfx".