One word: upgrade. I've seen cases of people running IV on a Pentium IV, but it isn't optimal enough for everyday use. The same goes for the video card; I had one and it does suck big time. Upgrading to at least Core 2 Duo and a 9500GT may help, but most of us here strongly recommend a quad-core, lots of RAM and an 8800GT.
thanks for the reply buddy , lets say i take what you are saying into consideration (which i certainly will) how much dow ($_$) do i have to apply to be able to get a quad-core , 4 gb ram and the 8800 gt ? if an upgrade is worth as much as buying a new computer , what specs would you recommend for me to be able to run gta4 with highest resolution and not get the slightest lag ?