it runs well on my computer (core 2 duo e6600 @ 3 ghz, 2gb ram and 8800gts Oc'd), FPS rarely drops below 25 fps on default settings @ 1440x900. I am running windows xp so maybe windows 7 requires a bit more ram. Your vid card should be getting more fps though and your cpu is more than enough. GTAiv loves quad cores. If you have dropped all the graphics settings, and still no cluck, I dont know what else ou can do?????
Right this is simple - you need to add a text file into the game dir and add the line inside (norenderlimit or similar) and then run the game - it will take out the artificial limits on the pc verison of the game and give you at least 30% boost in performance.
I have replied to another gta4 thread with exact info but right this moment cant remember and or find the old thread !>....trust me it will make a HUGE differrence and will let you play the game fine.
quick google to find gta 4 console commands will tell you all u need to know...