You should take a look at your audio drivers, a few people had very nice systems, but outdated audio drivers that would give them negative Sim Speed.
agreed. When playing multiplayer, i play with GFX low and sound low qual and I had a simspeed of like, 4. Then I updated audio drivers and at same settings run at sim speed 7ish. And I have a 8600 GT and onboard sound 
And even running low GFX, I have no graphical issues, when playing single player I put max GFX, there is noticable difference. but nothing to be missed.
Oh, I also run 3.0 ghz dual core, but I also have 4gigs of RAM :/ not sure if that'll help you at all.