Actually, turning off the pagefile can help in some cases.
The pagefile, it basically uses your HD as ram when your real ram is full, paging is much slower due to HD's being much slower than physical memory, and this can cause choppies/lag/etc,..
But, you need lots of ram to do this, and some games actually rely on the pagefile(Kotor complains if you don't have one..), so disabling it can cause problems, but in GTA IV's case, I think this might could help, if you have at least 4gb, for an XP system(x64, 32bit only supports 2-3gb), and about 8gb for Vista...(Alright, 4gb might be enough..)
But, it will not break your computer as some seem to think, if this breaks your computer, I hate to tell you, your computer was already broken..
Disabling the pagefile is harmless, but not guaranteed to help, since some apps/games depend on its existence, but the OS itself is quite capable of running without it, hence the option to disable it..
Edit: Just to clarify, what I mean by depending on the pagefile, basically they use an ass load of ram, more than most systems have, so they expect the pagefile to expand to hold the resources, they will work without one, but performance would actually be worse, but only if you didn't have the physical ram necessary to cover what they try to store..
And don't forget the OS still needs ram too.. Along with everything else, so it will probably take even more than I suggested above, consider those minimum specs..