yup+1 to skaz. to get smooth gameplay from gta 4 you really need a quad. if you dont have 1 then your gonna be stuck at minimum settings... turn the settings back down and everything should normalize. you could try adjusting the render ahead limit form 2-3 or from 2-1 to see if that helps. any more than 1 point will actually cause more issues. but its worth trying.(check your nvidia panel for the render ahead limit in the games profile)
How do you change the render ahead?