Valve just updated their Hardware Survey
again, which I think is a very interesting resource of information. It's a list of statistics collected from the hardware of their users.
Interesting IMO: Nobody actually uses Windows Vista (only 5 percent of all users). Most people only have 128 or 256 MB of VRAM. After English, the next most used language is German. And: Only 1% of all users have DirectX 10 systems.