There's lots of info on circuits old and new here:
http://theracingline.net/racingcircuits/racingcircuits/
I also have a plug in for Google Earth called Motorsport.kmz which gives the location for most of the circuits on the above website. I can't remember where I got it from but it's very useful
Edit: Google earth plug ins can be found here:
http://www.aerialf1.com/