Getting demographics

I was just wondering if there was a good place to start looking at demographics for the game industry. I know you generally need to pay for them but I find that they mostly center around the console game market. Although statistics for general PC usage could be useful it would be nice if they through in some stats on computer games such as average PC specs for people playing games.

Any time I read over an article of say a game review they always assume that I know what an average computer is for a game player when I don't think that the reviewer really knows.

Alc