A GFX card is a must! I personally have a CV3D with its scandoubler module and it`s wonderful :-) Other cards that support the same config are PicassoIV and CV64 (both offer a scandoubled amiga video).
The only reason that I don`t get a mediator is that I don`t want to lose the scandoubled video (common VGA/RGB-out, you need just one cable to the monitor, no switchers etc.)
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Vapor Genie