All of the options listed (except Dwarven Forge) require work that is well worth it if you end up using the terrain. I've gotten very fond of the cardstock models (though I am putting them on matboard or foamcore to help them last longer). There are a lot of great sets for them now. My major investment for it has been a paper cutter (saves the calluses *grin*). My partner got a load of the Hirst Arts molds (courtesy of myself and a friend) for Christmas and has been happily humming around dripping plaster on his workbench on weekends ever since

. The molds are great, you can do a LOT with them, and they are very sturdy (dental plaster is good stuff); he also uses them for wargaming terrain.
So in essence, we use a combination of 'cardstock' and plaster molds. My suggestion is to start with what you can afford, and what you can store (storage gets to be a big issue with 3D terrain). The cardstock models require a color printer, cardstock, a backing if you want extra sturdy, a cutter of some kind, and an adhesive. The plaster models require the molds (solid investment and will last a LONG time), the plaster, adhesive, paints, and terrain fuzzies (grass, sand, etc.). Foam is as someone said good for outside terrain, but it's hard to do too much creative with it unless you are a sculptor with a foam-cutter *grin*, and Dwarven Forge is beautiful, but very expensive, especially if you are willing to make your own (the plaster molds are just as good looking and nearly as hard).