The Vista Experience Index can't actually go above 5.9 right now for some reason. I think MS said it was something to do with no hardware existing to allow for it so it got capped to avoid confusion and erratic results.
You can change your Index to anything you like if you go to C:\Windows\Performance\WinSat\Datastore and open the XML file you'll find in there. Edit Line 12 to a value of your choice! Although I don't know if putting in a negative index score or something huge will break it because I haven't tried it myself, just something a colleague told me.