GTA IV is very CPU dependant, and even though yours is very fast, it still won't cut it for higher settings.
As for the shadows, the shadows are very heavy and take away a lot of performance.
There's not much you can do, I guess.
Are you already using commandline
And what patch are you using? If you're using patch 7, reinstall the game and use patch 4 as that one is the best one performance-wise.
But if you're still using patch 7, turn off Night Shadows as those kill performance a lot and they are ugly anyway.
Also, turning off Definition makes it look smoother, if you'd ask me.