GTA4 can be like that. A hand holding 'feature' that didn't allow for new cards with more RAM sometime down the line = Fail. The game thought my 7970 had something like 128MB or so VRAM.
The fix is a commandline text. Open notepad from within GTA4's main directory and name the file commandline.txt
Enter the following and save the text to the main GTA4 directory. Do the same in the EFLC main directory if you have that. You may not need the resolution commands with the other two, but I use them anyway.