OK - I've tried the VirtualBox solution now. The newest version of VB I could install was 3.0.12 (because I have Tiger), but it works like a charm...
Scrolling on Giana Sisters is a bit jerky, though. I thought that might have something to do with 3D acceleration. I know the amiga emulator doesn't do 3D, but many 2D games on the mac actually use OpenGL for hardware acceleration even in 2D. Might it be similar on the win platform? Anyway, turning on 3D acceleration didn't help... but it also says in the VB docs that 3D support is experimental for this version. Has anyone any experience with this? Can VB 3.1 provide smoother scrolling on WinUAE? Or should I experiment a little with WinUAE settings instead? The jerkyness doesn't ruin gameplay, though, it's just a little bit annoying.
Thanks anyway...