wow that was a helpful post! and completely on topic!
anyway,
to get GTA IV to display in 1080p go to display settings > HDMI > Custom Settings and untick everything except 1080p.
i myself, and many others have noticed the game runs alot smoother with this setup, i cant play the game in 720, its instantly noticeable (nothing to do with graphic, seems to run smoother perhaps more FPS, im not sure)
The best answer of course is having a play around yourself! see what suits your setup best and stick with that, have fun with it