I don't have any problems with GTAIV on my 8500GT. Download EVGA Precision and (if you have a cooler, it can be a stock cooler like mine) overclock it to:
Core Clock: 670Mhz
Memory Clock: 450Mhz
That is the maximum and if you overclock more, you'll have some problems in games like artifacts, etc. With those settings I get over 29FPS in the benchmark, average of 15/28FPS outside and some outraging 60FPS inside.