For requirements, I would assume minimum things required to run you game.
It will depend on what technologies you use to develop your games.
For example, if you use specific APIs, such as D3D9 or OpenGL 3.0, you will need to specific that, because certain APIs are not downward compatible.
If you build with windows with .net framework, you will also need to specific that as well.
For performance requirements, trying on different machines are pretty much the best way to go, though you may be able to measure the related performance by calculation.
for example, if your current video card runs at 60 fps, and another computer with the similar configuration, except the video card is half speed of yours, should run about the half speed of your machine. Again, this is just an estimate, many others properties will hit the performance differently, but you can use that base info to test your game on targeted devices.
Hope that helps, and anyone is welcome to correct me for any reasons.