If your monitor is one of the few that can handle the Amiga's native video scan rates (15KHz horizontal and 50Hz vertical), then you just need an RGB to VGA adapter. Unfortunately, I had trouble with the straight through pin adapters that are commonly available. There are a number of older adapters (the "Commodore silverbox") that have some additional circuitry in them to deal with genlocks that will definitely work.
If your monitor is a standard VGA monitor (31KHz and 56Hz), you need an Indivision ECS, mounted internally.
