Your GPU does meet the 'recommended' requirements for GTA IV, but the CPU does not. You will get very low fps regardless of resolution or graphical settings as a result.
I don't know what 'recommended' refers to in terms of resolution/settings, but for a PC it was probably 720p + Medium settings back in 2008.
Ignoring the CPU for a moment, that GPU will still struggle a bit with High settings at 720p, at with 1080p you have no hope of a consistent 40 fps on High.
Maybe Medium settings at 720p (with a more powerful CPU) you will have a chance of 40 to 45fps.