It's nor an issue with your Graphics card or nVidia Vs ATI/AMD. It's your processor. The graphics card you have is OK. Just OK. Not great. Drop the detail to medium across the board do NOT enable Night Shadows as they kill the performance.
Second, you will want to create a commandline.txt file in your GTAIV directory.
Inside that file put:
Also, enable Vsync in graphic options.
If this doesn't solve your problem then nothing will apart from lowering the settings more or upgrading your CPU. GTAIV Is not a good game to benchmark performance on. The coding is horrible as it's a bad port from Xbox 360.