I play Crusin at full speed...
no sound, cheats on, then ~ and underclock cpu0 to about 20 - 30 percent...
its still slow selecting the car...
but after that its MUCH better! I get 100% (about 80% with sound) on a 2.4 ghz machine
Just to let you know...
btw, the controls SUCK... you can do it a little with a mouse.. but forget the keyboard completely