GTA IV is an incredibly CPU intensive game. You need at least a dual-core CPU of 2 GHz to play it on low settings and a Quad-Core or a very fast Dual-Core CPU (3 Ghz+) with lots of L2 Cache to run it on medium settings or above.
As I've stated in my previous comment, your CPU is what's holding you back. If you want to play GTA IV on medium settings or higher you need a very fast Dual-Core CPU, or for optimum performance a Tri-Core or Quad-Core is recommended.
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