Unfortunately, OpenGL support can be a nightmare for a lot of people, because a number of the video card manufacturers don't put as much effort into the development of their OpenGL drivers as they do for their DirectX stuff, which is a sad state of affairs in itself. The other problem is that the "bog-standard" Windows drivers are often much older than what is currently available because the WHQL certification process is long and expensive (and a hassle), so video card manufacturers will only usually certify the original release version, and then just require people to download and install the latest drivers on their own.

As for your second question, the short answer is that the only way to know what hardware requirements your players will need is to just develop it and see. Obviously, there will be minimums, based on the level of technology you choose to require as mandatory (i.e., if you require Shader 2.x support, then any video hardware without that support won't work). During the development process, you will begin to get an idea what kind of iron your game is going to take to run it, and how well. Of course, part of the later development process for many games includes optimizations; you'll find all kinds of ways to speed up and slim down your game, and that will also impact the minimum requirements.