The best help for running that game on that system would be a new video card and some more ram. Your entire system is really rather bad for current games and especially that one but your card is the weakest link as it was never a gaming card and is getting quite old. Something like an HD4650 or 9500GT and adding another gig of ram to your system will help a lot and be fairly cheap. It won't do miracles because your processor is still poor but it should still improve performance greatly in that game and even more so in other games that aren't as cpu dependent.
Saints Row should run better on that system than GTA4 but still not very well. For something like GTA that actually will run pretty well on your current system I would recommend Mafia;http://www.metacritic.com/games/platforms/pc/mafia
It's good stuff and a sequel should be out soon if you ever get a system capable of playing it.
Some other 3rd person sandbox type PC games similar to GTA to varying degrees are Scarface, Bully, Godfather 1 & 2, Simpsons: Hit & Run, Mercenaries 2, Just Cause 1 & 2, Red Faction Guerrilla, Prototype and even maybe the Assassins Creed games. When you look up these games keep in mind that the older they are the more likely they are to run well on your system.