GTA 4 is a CPU intensive game instead of just a GPU intensive game like most of the games nowadays...which means that you need 2 things, a fast CPU and a powerful graphic card.
As far as I tested the game will run well on any pc that has:
-> at least 3 gigs of RAM for a 32bit os or 4 gigs of ram for a 64 bit os (like VISTA 64bit) that's because the game takes up to 1,5 gigs of ram and you need to have that free.
->a fast video card like a Nvidia 8800GT or a ATI 3870 or better with at least 512 megs of VRAM (the game on minimal settings looks awful, but an 8800GT gives you the chance of setting everything on high, 512 megs isn't enough to max the settings out, but a low end video card with a lot of low speed memory and/or shared memory is no use for this game.)
->a fairly new, fast CPU: At least a 3,0 GHz AMD Athlon x2 or a 2,66 GHz Intel Core 2 duo. If running on vista or Windows 7 a quad core CPU might improve gameplay regardless of CPU speed (any 2,0GHz AMD or Intel quad core is enough)
the lack of any of this things will make the game umplayable or just barely playable...
Keep in mind that the cpu will be loaded during all gameplay and so does the videocard, which makes it quite rough for a laptop (and only a gaming laptop would play it well, keep in mind that a mobile GPU has on average half the performance of the desktop counterpart, which make a GT230m almost certainly no match for a desktop 8600GT which is only capable of low to medium settings.)