Also on top of that, make a commandline if you havent already as that will boost fps.
STEP ONE
make a new .TXT and name it " Commandline.txt "
then copy and paste the following into it exactly how it is
-norestriction
-nomemrestrict
-availablevidmem "1.1"
-percentvidmem "100"
put the .txt into the main directory of GTA IV where the GTAIV.exe is found
and then run the game as normal this already should help performance a great deal on any pc.
Dave