Welcome to the party.
I am afraid such is the case with just about everyone. This fact is far more evident seeing as how it is a good game that people want to play but can't.
I have an i5 2500k clocked at 4.50GHz currently with 16GB of DDR3 and dual 6970s in CrossfireX @ 930/1440, and when I try maxing it out my frame rate crumbles. It doesn't actually do too much better when I drop the settings.
I am afraid GTA IV is a lost cause on PC. But...
There is one little fix that does seem to help. Override every single graphics option (such as AA, AF, and Tessellation) in your Nvidia control panel and turn them off completely. Then, go in game and set the resolution at 1280x720 (no higher). After that, be sure and turn all settings to medium and shadows OFF. Lastly, turn the view and detail distance down to 20 or lower, and vehicle density down to 30. What you end up with is essentially the console version, maybe a tad better. And it should run at well over 50 frames per second (almost double the console version) at these settings.
It might do wonders to overclock your CPU also. You may also try using Game Booster, seeing as how some people actually get a boost of 2 or 3 frames per second from using it.
Good luck.