Well it's not your rig, that's for sure
This is what I've found, maybe it helps:
Go to your GTA IV folder, and see if there is a .txt file called "commandline". If there is, and it says something that starts with "memrestrict", close it and make a backup copy. Then, clear the command line, then paste this: -noprecache -nomemrestrict -norestrictions
That's part of my commandline, I made my GTA IV work properly with that. If this doesn't work for you, then go back to your original commandline (this is why the backup copy is important).
Which patch are you on btw?