It depends on what software you plan on using, what game you are developing for, how much interaction you have with code monkeys and what your poly budget is for enviromental art. Most 3D apps worth their salt have displacement maps, which is a good place to start but can be expensive on the poly budget.
I use 3D Studio Max8 to do my terrain generation. A brief overview of my workflow is listed in this thread
, you might find it handy. Programs like Vue
look nice but I haven't offically tried them out to see what thier range actually is.