Well there are a few options
One is that you get a Picasso IV graphics card
Another is that you get a multisync LCD, able to sync down to 15kHz
Another is that you get a scandoubler/ flickerfixer and use the Amiga's onboard garphics
Another is that you get a mediator and a PCI gfx card (will still need multisync monitor/or scandoubler+FF to display all screenmodes)
I think that covers it. I use a commodore monitor with my A4000, so I don't have the same problem at the moment - perhaps someone else can fill in the gaps / correct anything I've got wrong there....