Here is what you need to do open a support ticket with rock star with your pc specs listed they will then create a command set for you to put in the launcher

Lol, okay. I had this exact same problem with my Crossfired 6950's. What I had to do to fix it was to go to Steam - Library - GTA IV - Right Click and go to Properties - Click Set Launch Options and type -norestriction . That fixed it for me and I got good frame rates and could adjust whatever I wanted as high as I wanted. Hopefully it works for you as well

I have been quite lucky with GTA running it on full settings, no lag with a gtx 660 however it is a really bad port so i know a lot of people have problems running it.

I Had the same problem there is 2 settings you have to turn down or off, just look about in the settings mine runs at about 60fps since ive done it.