256mb VRAM is far far far from enough to run GTA 4 smoothly, it would probably run at 10-20 fps with everything set to low + tweaked to the minimum in config files. What are the rest of your computer specs? GTA 4 is a still a pretty demanding game despite being released for a long time.
EDIT: Although this card meets the minimum requirements of the game, i dont suggest trying if u want a smooth and enjoying experience