The tip is, that sometimes on quad core processors GTA IV and other programs tend torun on only 2 cores and you have to enable the rest of the cores for your GTA IV not to lag.
LTL went from 15 -70 fps if i got him right.
For those who don`t know, what you have to do is go to task manager ( when IV is running) right click on GTAIV.exe, click "set affinity" and then you just check the empty boxes and voila you have lag free gaming.
However if you are a user of windows Vista/7 it might say "Blah blah you have no access and whatnot".
DON`T DESPAIR.
What you have to do is go to control panel and go to User Account Control and disable it by pulling the slider allllllllll the way down.
Then repeat step number 1 and voila, lag free gaming.
If it still doesn`t work hell will freeze XD