Here is a solution I've found to my horrible framerate issues with my core2duo 6300, and geforce 8800GTS.
When I start the game and load a scenario, the initial framerate is unbelievably slow ~5fps.
If I open the in game video options panel, and check off the "use one cpu" box and click apply, then go back in to the game, the framerate has increased to around 10-15fps.
Finally, if I return to the ingame options panel and uncheck the "use one cpu" box and click apply, when I return to the game the framerate again increases to over 20fps or higher... perfectly playable.
So long as I don't restart the game, loading a new scenario will remain at a good framerate.
This seems like very strange behavior, but perhaps the game is not properly initialising it's multithreading code when it starts up. Perhaps resetting the system once a map is loaded is forcing it to properly recognise the cpu.
Hope this helps someone... I'm curious to know if this situation occurs on anyone else's machine.