Terrain polygon count

I'm wanting to make a next-gen game, but me and my team members are new to game development. I would like to know what would be a good place to start on creating terrains if we are trying to creating a next-gen game. We want to terrains to have as much detail as possible without slowing down the framerate? The type of computer we are using to develop our project have a 256 graphic card, with about 2 gigs of Ram memory, a AMD 3400 series with ATI tech. If youcan answer this it would help us out some. Thanks