A weak processor on a high end card will definately be a problem.
Older games are less stressful so they work. The newer ones wont. I ran prototype at that resolution and my gpu is kinda weaker than yours, but no stutters at all! You need processing power for sure! GTA4 must run real bad rite ? I play GTA4 1600x1200 Med Textures High render 20 view/detail/traffic and 3 shadow. Almost console settings but at 1600x1200 ... n boi ... it runs fine!