It's only going to be playable at the lowest possible settings. You didn't say what your CPU is. GTA4 is a very CPU heavy game, it is one of the very few games where your CPU is possibly more important than your graphics card in determining performance. Most dual core CPUs perform horribly in that game. If you only have 1GB of RAM on your system, that probably won't be enough. I've seen GTA4 use up to 2GB of RAM on its own. It won't run well on less than 1GB.