I'm not very familiar with the ATI control panel, but you should be able to create a performance profile for GTAIV.exe and there force the 3D application settings. The problem is, GTAIV for some reason doesn't flip the 2D/3D performance switch, so you're currently running the game with integrated GPU.
Alternately, what apparently worked for some people is use of norestrictions command line switch
http://www.gtaforums.com/index.php?showtopic=458859