Well you have same CPU as me but i have mine overclocked now to 3.0 ghz
I have only 2gig ram
Your graphics card is better than mine and i have geforce 8500 gt overclocked
And I RUN GTA IV on 640*480 resolution BUT everything on High settings and view distance at 15
Check out My GTA IV FPS BOOST and youl get even better results
We all know how stupidly badly coded GTA IV is, so the minimum requirements are a bad joke. A lot of users with way more powerful PC's than the minimum requirements cannot get playable frame rates from that game.
So... beware. At least for me, this is a hopeless case. I gave it up a long time ago.