Hello, my system has 4 GB RAM, AMD Athlon 64 X2 4200+ proccessor, and NVidia GeForce 8600 GT. I bought GTA 4, but when I play it is really laggy, and after about 10 seconds, half the city becomes invisible! i.e. No roads, buildings people, etc. All I can see are the lights, cars and some buildings in the distance! What kind of upgrades do I need to make this game work decently? Is it even worth it?!