A quick update, I was just doing some benchmarking with fraps, and it seems that I am easily able to play with maximum detail + HDR at 1024x768 at a steady 30fps, however I have always used my native resolution of 1440x900 which obviously drags FPS down alot.
So at 1440x900 I was getting a steady average of 32fps at max/4xAA/no HDR.
and at 1024x768 I was getting a steady average of 31fps at max/HDR.
However as you know, the 7*** series cards cannot support both HDR and AA running together at once, so while I may be able to play with HDR, the cars and envirnments without AA support look lagged and unappealing. So technically for me I am glad that I don't run with HDR enabled as AA makes the cars look far more realistc.
Hope that helps.
EDIT: Also, at 2x AA I would get an average of about 38fps, and with no AA and no HDR an average of 42FPS.