i see you have an X2......could it be that having two cpus is messing up the game play? if windows hasnt been sorted properly and it doesnt no what to do with two cpus? just a guess, a cpu problem would describe the stuttery/ sped up effect.
you've definately got enough PSU there, so it cant be that.
have you made sure that you've not inadvertantly set high levels of aa and af in the drivers?
are you aware of cool and quiet for AMD64 cpus? it dynamically adjusts cpu speed depending on the cpu load, it does this by adjusting the cpu multiplier. if youve installed this, you need to have power management set to "minimal power management" any other setting will lock the cpu at a certain speed. if youve installed it, and its not at the "minimal power management" when you start up your computer it will start in a low power mode....basically the cpu multiplier will be at the lowest set in the cool n quiet driver.....typically x5 or x4 so your cpu starts up at say 1Ghz, and becuase your not in the right power mode in windows it'll stay at 1Ghz until you change it back to "minimal"
this could also be your problem....if you have it installed and set up right, the fact that the cpu speeds up n down depending on load could be why your older games are seemingly sped up? older games dont present alot of load in comparison (tho NFSU is pretty new) so when the games at a quiet part the cpu is slowing down, then is being asked to speed up again when the action gets more intense and this speeding up and slowing down is playing havok with the game engine . just a shot in the dark