GTAIV runs at 60fps with almost everything maxed out at 1920x1200 on my PC. The only thing I have to dial down is texture settings, which I keep at medium, because my videocard only has 1 gig of memory. GTAIV is the most retardly video memory demanding game in recent memory. Supposedly, it requires at least a 1.5 gig frame buffer to have everything maxed out with the texture setting at high.
I'm guessing that most of the performance issues on the PC are due to inefficient streaming from a hard drive. Before I started using a SSD, I experience a lot more stuttering with GTA4. Now it's completely smooth. Rockstar has always been really terrible at optimizing streaming on the PC ports. The original GTA3 stuttered much more on a PC than it did on a PS2 which was only streaming from a slow DVD drive. Most people could get GTA3 running at 60fps but it would annoying hitch for a moment every couple of seconds.